graphql-engine/server/src-lib/Hasura/Backends/MSSQL/FromIr/Query.hs

{-# LANGUAGE ViewPatterns #-}

-- | This module defines translation functions for queries which select data.
-- Principally this includes translating the @query@ root field, but parts are
-- also reused for serving the responses for mutations.
module Hasura.Backends.MSSQL.FromIr.Query (fromQueryRootField, fromSelect) where

import Control.Monad.Validate
import Data.HashMap.Strict qualified as HM
import Data.Map.Strict (Map)
import Data.Map.Strict qualified as M
import Data.Proxy
import Database.ODBC.SQLServer qualified as ODBC
import Hasura.Backends.MSSQL.FromIr
  ( Error (..),
    FromIr,
    NameTemplate (..),
    generateAlias,
  )
import Hasura.Backends.MSSQL.Instances.Types ()
import Hasura.Backends.MSSQL.Types.Internal as TSQL
import Hasura.Prelude
import Hasura.RQL.IR qualified as IR
import Hasura.RQL.Types.Column qualified as IR
import Hasura.RQL.Types.Common qualified as IR
import Hasura.RQL.Types.Relationships.Local qualified as IR
import Hasura.SQL.Backend

-- | This is the top-level entry point for translation of Query root fields.
fromQueryRootField :: IR.QueryDB 'MSSQL Void Expression -> FromIr Select
fromQueryRootField =
  \case
    (IR.QDBSingleRow s) -> fromSelect IR.JASSingleObject s
    (IR.QDBMultipleRows s) -> fromSelect IR.JASMultipleRows s
    (IR.QDBAggregation s) -> fromSelectAggregate Nothing s

fromSelect ::
  IR.JsonAggSelect ->
  IR.AnnSelectG 'MSSQL (IR.AnnFieldG 'MSSQL Void) Expression ->
  FromIr TSQL.Select
fromSelect jsonAggSelect annSimpleSel =
  case jsonAggSelect of
    IR.JASMultipleRows ->
      guardSelectYieldingNull emptyArrayExpression <$> fromSelectRows annSimpleSel
    IR.JASSingleObject ->
      fmap (guardSelectYieldingNull nullExpression) $
        fromSelectRows annSimpleSel <&> \sel ->
          sel
            { selectFor =
                JsonFor
                  ForJson {jsonCardinality = JsonSingleton, jsonRoot = NoRoot},
              selectTop = Top 1
            }
  where
    guardSelectYieldingNull :: TSQL.Expression -> TSQL.Select -> TSQL.Select
    guardSelectYieldingNull fallbackExpression select =
      let isNullApplication = FunExpISNULL (SelectExpression select) fallbackExpression
       in emptySelect
            { selectProjections =
                [ ExpressionProjection $
                    Aliased
                      { aliasedThing = FunctionApplicationExpression isNullApplication,
                        aliasedAlias = "root"
                      }
                ]
            }

-- | Top/root-level 'Select'. All descendent/sub-translations are collected to produce a root TSQL.Select.
fromSelectRows :: IR.AnnSelectG 'MSSQL (IR.AnnFieldG 'MSSQL Void) Expression -> FromIr TSQL.Select
fromSelectRows annSelectG = do
  selectFrom <-
    case from of
      IR.FromTable qualifiedObject -> fromQualifiedTable qualifiedObject
      IR.FromIdentifier identifier -> pure $ FromIdentifier $ IR.unFIIdentifier identifier
      IR.FromFunction {} -> refute $ pure FunctionNotSupported
  Args
    { argsOrderBy,
      argsWhere,
      argsJoins,
      argsTop,
      argsDistinct = Proxy,
      argsOffset,
      argsExistingJoins
    } <-
    runReaderT (fromSelectArgsG args) (fromAlias selectFrom)
  fieldSources <-
    runReaderT
      (traverse (fromAnnFieldsG argsExistingJoins stringifyNumbers) fields)
      (fromAlias selectFrom)
  filterExpression <-
    runReaderT (fromGBoolExp permFilter) (fromAlias selectFrom)
  let selectProjections = map fieldSourceProjections fieldSources
  pure $
    emptySelect
      { selectOrderBy = argsOrderBy,
        selectTop = permissionBasedTop <> argsTop,
        selectProjections,
        selectFrom = Just selectFrom,
        selectJoins = argsJoins <> mapMaybe fieldSourceJoin fieldSources,
        selectWhere = argsWhere <> Where [filterExpression],
        selectFor =
          JsonFor ForJson {jsonCardinality = JsonArray, jsonRoot = NoRoot},
        selectOffset = argsOffset
      }
  where
    IR.AnnSelectG
      { _asnFields = fields,
        _asnFrom = from,
        _asnPerm = perm,
        _asnArgs = args,
        _asnStrfyNum = stringifyNumbers
      } = annSelectG
    IR.TablePerm {_tpLimit = mPermLimit, _tpFilter = permFilter} = perm
    permissionBasedTop =
      maybe NoTop Top mPermLimit

mkNodesSelect :: Args -> Where -> Expression -> Top -> From -> [(Int, (IR.FieldName, [FieldSource]))] -> [(Int, Projection)]
mkNodesSelect Args {..} foreignKeyConditions filterExpression permissionBasedTop selectFrom nodes =
  [ ( index,
      ExpressionProjection $
        Aliased
          { aliasedThing =
              SelectExpression $
                emptySelect
                  { selectProjections = map fieldSourceProjections fieldSources,
                    selectTop = permissionBasedTop <> argsTop,
                    selectFrom = pure selectFrom,
                    selectJoins = argsJoins <> mapMaybe fieldSourceJoin fieldSources,
                    selectWhere = argsWhere <> Where [filterExpression] <> foreignKeyConditions,
                    selectFor =
                      JsonFor ForJson {jsonCardinality = JsonArray, jsonRoot = NoRoot},
                    selectOrderBy = argsOrderBy,
                    selectOffset = argsOffset
                  },
            aliasedAlias = IR.getFieldNameTxt fieldName
          }
    )
    | (index, (fieldName, fieldSources)) <- nodes
  ]

--
-- The idea here is that LIMIT/OFFSET and aggregates don't mix
-- well. Therefore we have a nested query:
--
-- select sum(*), .. FROM (select * from x offset o limit l) p
--
-- That's why @projections@ appears on the outer, and is a
-- @StarProjection@ for the inner. But the joins, conditions, top,
-- offset are on the inner.
--
mkAggregateSelect :: Args -> Where -> Expression -> From -> [(Int, (IR.FieldName, [Projection]))] -> [(Int, Projection)]
mkAggregateSelect Args {..} foreignKeyConditions filterExpression selectFrom aggregates =
  [ ( index,
      ExpressionProjection $
        Aliased
          { aliasedThing =
              safeJsonQueryExpression JsonSingleton $
                SelectExpression $
                  emptySelect
                    { selectProjections = projections,
                      selectTop = NoTop,
                      selectFrom =
                        pure $
                          FromSelect
                            Aliased
                              { aliasedAlias = aggSubselectName,
                                aliasedThing =
                                  emptySelect
                                    { selectProjections = pure StarProjection,
                                      selectTop = argsTop,
                                      selectFrom = pure selectFrom,
                                      selectJoins = argsJoins,
                                      selectWhere = argsWhere <> Where [filterExpression] <> foreignKeyConditions,
                                      selectFor = NoFor,
                                      selectOrderBy = mempty,
                                      selectOffset = argsOffset
                                    }
                              },
                      selectJoins = mempty,
                      selectWhere = mempty,
                      selectFor =
                        JsonFor
                          ForJson
                            { jsonCardinality = JsonSingleton,
                              jsonRoot = NoRoot
                            },
                      selectOrderBy = mempty,
                      selectOffset = Nothing
                    },
            aliasedAlias = IR.getFieldNameTxt fieldName
          }
    )
    | (index, (fieldName, projections)) <- aggregates
  ]

fromSelectAggregate ::
  Maybe (EntityAlias, HashMap ColumnName ColumnName) ->
  IR.AnnSelectG 'MSSQL (IR.TableAggregateFieldG 'MSSQL Void) Expression ->
  FromIr TSQL.Select
fromSelectAggregate
  mparentRelationship
  IR.AnnSelectG
    { _asnFields = (zip [0 ..] -> fields),
      _asnFrom = from,
      _asnPerm = IR.TablePerm {_tpLimit = (maybe NoTop Top -> permissionBasedTop), _tpFilter = permFilter},
      _asnArgs = args,
      _asnStrfyNum = stringifyNumbers
    } =
    do
      selectFrom <- case from of
        IR.FromTable qualifiedObject -> fromQualifiedTable qualifiedObject
        IR.FromIdentifier identifier -> pure $ FromIdentifier $ IR.unFIIdentifier identifier
        IR.FromFunction {} -> refute $ pure FunctionNotSupported
      -- Below: When we're actually a RHS of a query (of CROSS APPLY),
      -- then we'll have a LHS table that we're joining on. So we get the
      -- conditions expressions from the field mappings. The LHS table is
      -- the entityAlias, and the RHS table is selectFrom.
      mforeignKeyConditions <- fmap (Where . fromMaybe []) $
        for mparentRelationship $
          \(entityAlias, mapping) ->
            runReaderT (fromMapping selectFrom mapping) entityAlias
      filterExpression <- runReaderT (fromGBoolExp permFilter) (fromAlias selectFrom)
      args'@Args {argsExistingJoins} <-
        runReaderT (fromSelectArgsG args) (fromAlias selectFrom)
      -- Although aggregates, exps and nodes could be handled in one list,
      -- we need to separately treat the subselect expressions
      expss :: [(Int, Projection)] <- flip runReaderT (fromAlias selectFrom) $ sequence $ mapMaybe fromTableExpFieldG fields
      nodes :: [(Int, (IR.FieldName, [FieldSource]))] <-
        flip runReaderT (fromAlias selectFrom) $ sequence $ mapMaybe (fromTableNodesFieldG argsExistingJoins stringifyNumbers) fields
      let aggregates :: [(Int, (IR.FieldName, [Projection]))] = mapMaybe fromTableAggFieldG fields
      pure
        emptySelect
          { selectProjections =
              map snd $
                sortBy (comparing fst) $
                  expss
                    <> mkNodesSelect args' mforeignKeyConditions filterExpression permissionBasedTop selectFrom nodes
                    <> mkAggregateSelect args' mforeignKeyConditions filterExpression selectFrom aggregates,
            selectTop = NoTop,
            selectFrom =
              pure $
                FromOpenJson $
                  Aliased
                    { aliasedThing =
                        OpenJson
                          { openJsonExpression = ValueExpression $ ODBC.TextValue "[0]",
                            openJsonWith = Nothing
                          },
                      aliasedAlias = existsFieldName
                    },
            selectJoins = mempty, -- JOINs and WHEREs are only relevant in subselects
            selectWhere = mempty,
            selectFor = JsonFor ForJson {jsonCardinality = JsonSingleton, jsonRoot = NoRoot},
            selectOrderBy = Nothing,
            selectOffset = Nothing
          }

--------------------------------------------------------------------------------
-- GraphQL Args

data Args = Args
  { argsWhere :: Where,
    argsOrderBy :: Maybe (NonEmpty OrderBy),
    argsJoins :: [Join],
    argsTop :: Top,
    argsOffset :: Maybe Expression,
    argsDistinct :: Proxy (Maybe (NonEmpty FieldName)),
    argsExistingJoins :: Map TableName EntityAlias
  }
  deriving (Show)

fromSelectArgsG :: IR.SelectArgsG 'MSSQL Expression -> ReaderT EntityAlias FromIr Args
fromSelectArgsG selectArgsG = do
  let argsOffset = ValueExpression . ODBC.IntValue . fromIntegral <$> moffset
  argsWhere <-
    maybe (pure mempty) (fmap (Where . pure) . fromGBoolExp) mannBoolExp
  argsTop <-
    maybe (pure mempty) (pure . Top) mlimit
  -- Not supported presently, per Vamshi:
  --
  -- > It is hardly used and we don't have to go to great lengths to support it.
  --
  -- But placeholdering the code so that when it's ready to be used,
  -- you can just drop the Proxy wrapper.
  let argsDistinct = Proxy
  (argsOrderBy, joins) <-
    runWriterT (traverse fromAnnotatedOrderByItemG (maybe [] toList orders))
  -- Any object-relation joins that we generated, we record their
  -- generated names into a mapping.
  let argsExistingJoins =
        M.fromList (mapMaybe unfurledObjectTableAlias (toList joins))
  pure
    Args
      { argsJoins = toList (fmap unfurledJoin joins),
        argsOrderBy = nonEmpty argsOrderBy,
        ..
      }
  where
    IR.SelectArgs
      { _saWhere = mannBoolExp,
        _saLimit = mlimit,
        _saOffset = moffset,
        _saOrderBy = orders
      } = selectArgsG

--------------------------------------------------------------------------------
-- Conversion functions
fromQualifiedTable :: TableName -> FromIr From
fromQualifiedTable schemadTableName@(TableName {tableName}) = do
  alias <- generateAlias (TableTemplate tableName)
  pure
    ( FromQualifiedTable
        ( Aliased
            { aliasedThing = schemadTableName,
              aliasedAlias = alias
            }
        )
    )

-- | Translate an 'AnnBoolExpFld' within an 'EntityAlias' context referring to the table the `AnnBoolExpFld` field belongs to.
--
-- This is mutually recursive with 'fromGBoolExp', mirroring the mutually recursive structure between 'AnnBoolExpFld' and 'AnnBoolExp b a' (alias of 'GBoolExp b (AnnBoolExpFld b a)').
fromAnnBoolExpFld ::
  IR.AnnBoolExpFld 'MSSQL Expression ->
  ReaderT EntityAlias FromIr Expression
fromAnnBoolExpFld =
  \case
    IR.AVColumn columnInfo opExpGs -> do
      expression <- fromColumnInfoForBoolExp columnInfo
      expressions <- traverse (lift . fromOpExpG expression) opExpGs
      pure (AndExpression expressions)
    IR.AVRelationship IR.RelInfo {riMapping = mapping, riRTable = table} annBoolExp -> do
      selectFrom <- lift (fromQualifiedTable table)
      foreignKeyConditions <- fromMapping selectFrom mapping
      whereExpression <-
        local (const (fromAlias selectFrom)) (fromGBoolExp annBoolExp)
      pure
        ( ExistsExpression
            emptySelect
              { selectOrderBy = Nothing,
                selectProjections =
                  [ ExpressionProjection
                      ( Aliased
                          { aliasedThing = trueExpression,
                            aliasedAlias = existsFieldName
                          }
                      )
                  ],
                selectFrom = Just selectFrom,
                selectJoins = mempty,
                selectWhere = Where (foreignKeyConditions <> [whereExpression]),
                selectTop = NoTop,
                selectFor = NoFor,
                selectOffset = Nothing
              }
        )

-- | For boolean operators, various comparison operators used need
-- special handling to ensure that SQL Server won't outright reject
-- the comparison. See also 'shouldCastToVarcharMax'.
fromColumnInfoForBoolExp :: IR.ColumnInfo 'MSSQL -> ReaderT EntityAlias FromIr Expression
fromColumnInfoForBoolExp IR.ColumnInfo {ciColumn = column, ciType} = do
  fieldName <- columnNameToFieldName column <$> ask
  if shouldCastToVarcharMax ciType -- See function commentary.
    then pure (CastExpression (ColumnExpression fieldName) WvarcharType DataLengthMax)
    else pure (ColumnExpression fieldName)

-- | There's a problem of comparing text fields with =, <, etc. that
-- SQL Server completely refuses to do so. So one way to workaround
-- this restriction is to automatically cast such text fields to
-- varchar(max).
shouldCastToVarcharMax :: IR.ColumnType 'MSSQL -> Bool
shouldCastToVarcharMax typ =
  typ == IR.ColumnScalar TextType || typ == IR.ColumnScalar WtextType

--------------------------------------------------------------------------------
-- Sources of projected fields
--
-- Because in the IR, a field projected can be a foreign object, we
-- have to both generate a projection AND on the side generate a join.
--
-- So a @FieldSource@ couples the idea of the projected thing and the
-- source of it (via 'Aliased').

data FieldSource
  = ExpressionFieldSource (Aliased Expression)
  | JoinFieldSource JsonCardinality (Aliased Join)
  deriving (Eq, Show)

-- | Get FieldSource from a TAFExp type table aggregate field
fromTableExpFieldG :: -- TODO: Convert function to be similar to Nodes function
  (Int, (IR.FieldName, IR.TableAggregateFieldG 'MSSQL Void Expression)) ->
  Maybe (ReaderT EntityAlias FromIr (Int, Projection))
fromTableExpFieldG = \case
  (index, (IR.FieldName name, IR.TAFExp text)) ->
    Just $
      pure $
        ( index,
          fieldSourceProjections $
            ExpressionFieldSource
              Aliased
                { aliasedThing = TSQL.ValueExpression (ODBC.TextValue text),
                  aliasedAlias = name
                }
        )
  _ -> Nothing

fromTableAggFieldG ::
  (Int, (IR.FieldName, IR.TableAggregateFieldG 'MSSQL Void Expression)) ->
  Maybe (Int, (IR.FieldName, [Projection]))
fromTableAggFieldG = \case
  (index, (fieldName, IR.TAFAgg (aggregateFields :: [(IR.FieldName, IR.AggregateField 'MSSQL)]))) ->
    Just $
      let aggregates =
            aggregateFields <&> \(fieldName', aggregateField) ->
              fromAggregateField (IR.getFieldNameTxt fieldName') aggregateField
       in (index, (fieldName, aggregates))
  _ -> Nothing

fromTableNodesFieldG ::
  Map TableName EntityAlias ->
  IR.StringifyNumbers ->
  (Int, (IR.FieldName, IR.TableAggregateFieldG 'MSSQL Void Expression)) ->
  Maybe (ReaderT EntityAlias FromIr (Int, (IR.FieldName, [FieldSource])))
fromTableNodesFieldG argsExistingJoins stringifyNumbers = \case
  (index, (fieldName, IR.TAFNodes () (annFieldsG :: [(IR.FieldName, IR.AnnFieldG 'MSSQL Void Expression)]))) -> Just do
    fieldSources' <- fromAnnFieldsG argsExistingJoins stringifyNumbers `traverse` annFieldsG
    pure (index, (fieldName, fieldSources'))
  _ -> Nothing

fromAggregateField :: Text -> IR.AggregateField 'MSSQL -> Projection
fromAggregateField alias aggregateField =
  case aggregateField of
    IR.AFExp text -> AggregateProjection $ Aliased (TextAggregate text) alias
    IR.AFCount countType -> AggregateProjection . flip Aliased alias . CountAggregate $ case countType of
      StarCountable -> StarCountable
      NonNullFieldCountable name -> NonNullFieldCountable $ columnFieldAggEntity name
      DistinctCountable name -> DistinctCountable $ columnFieldAggEntity name
    IR.AFOp IR.AggregateOp {_aoOp = op, _aoFields = fields} ->
      let projections :: [Projection] =
            fields <&> \(fieldName, columnField) ->
              case columnField of
                IR.CFCol column _columnType ->
                  let fname = columnFieldAggEntity column
                   in AggregateProjection $ Aliased (OpAggregate op [ColumnExpression fname]) (IR.getFieldNameTxt fieldName)
                IR.CFExp text ->
                  ExpressionProjection $ Aliased (ValueExpression (ODBC.TextValue text)) (IR.getFieldNameTxt fieldName)
       in ExpressionProjection $
            flip Aliased alias $
              safeJsonQueryExpression JsonSingleton $
                SelectExpression $
                  emptySelect
                    { selectProjections = projections,
                      selectFor = JsonFor $ ForJson JsonSingleton NoRoot
                    }
  where
    columnFieldAggEntity col = columnNameToFieldName col $ EntityAlias aggSubselectName

-- | The main sources of fields, either constants, fields or via joins.
fromAnnFieldsG ::
  Map TableName EntityAlias ->
  IR.StringifyNumbers ->
  (IR.FieldName, IR.AnnFieldG 'MSSQL Void Expression) ->
  ReaderT EntityAlias FromIr FieldSource
fromAnnFieldsG existingJoins stringifyNumbers (IR.FieldName name, field) =
  case field of
    IR.AFColumn annColumnField -> do
      expression <- fromAnnColumnField stringifyNumbers annColumnField
      pure
        ( ExpressionFieldSource
            Aliased {aliasedThing = expression, aliasedAlias = name}
        )
    IR.AFExpression text ->
      pure
        ( ExpressionFieldSource
            Aliased
              { aliasedThing = TSQL.ValueExpression (ODBC.TextValue text),
                aliasedAlias = name
              }
        )
    IR.AFObjectRelation objectRelationSelectG ->
      fmap
        ( \aliasedThing ->
            JoinFieldSource JsonSingleton (Aliased {aliasedThing, aliasedAlias = name})
        )
        (fromObjectRelationSelectG existingJoins objectRelationSelectG)
    IR.AFArrayRelation arraySelectG ->
      fmap
        ( \aliasedThing ->
            JoinFieldSource JsonArray (Aliased {aliasedThing, aliasedAlias = name})
        )
        (fromArraySelectG arraySelectG)

-- | Here is where we project a field as a column expression. If
-- number stringification is on, then we wrap it in a
-- 'ToStringExpression' so that it's casted when being projected.
fromAnnColumnField ::
  IR.StringifyNumbers ->
  IR.AnnColumnField 'MSSQL Expression ->
  ReaderT EntityAlias FromIr Expression
fromAnnColumnField _stringifyNumbers annColumnField = do
  fieldName <- fromColumn column
  -- TODO: Handle stringifying large numbers
  {-(IR.isScalarColumnWhere isBigNum typ && stringifyNumbers == IR.StringifyNumbers)-}

  -- for geometry and geography values, the automatic json encoding on sql
  -- server would fail. So we need to convert it to a format the json encoding
  -- handles. Ideally we want this representation to be GeoJSON but sql server
  -- doesn't have any functions to convert to GeoJSON format. So we return it in
  -- WKT format
  if typ == (IR.ColumnScalar GeometryType) || typ == (IR.ColumnScalar GeographyType)
    then pure $ MethodApplicationExpression (ColumnExpression fieldName) MethExpSTAsText
    else case caseBoolExpMaybe of
      Nothing -> pure (ColumnExpression fieldName)
      Just ex -> do
        ex' <- fromGBoolExp (coerce ex)
        let nullValue = ValueExpression ODBC.NullValue
        pure (ConditionalExpression ex' (ColumnExpression fieldName) nullValue)
  where
    IR.AnnColumnField
      { _acfColumn = column,
        _acfType = typ,
        _acfAsText = _asText :: Bool,
        _acfOp = _ :: Maybe (IR.ColumnOp 'MSSQL), -- TODO: What's this?
        _acfCaseBoolExpression = caseBoolExpMaybe
      } = annColumnField

-- | This is where a field name "foo" is resolved to a fully qualified
-- field name [table].[foo]. The table name comes from EntityAlias in
-- the ReaderT.
fromColumn :: ColumnName -> ReaderT EntityAlias FromIr FieldName
fromColumn column = columnNameToFieldName column <$> ask

--  entityAlias <- ask
--  pure (columnNameToFieldName column entityAlias -- FieldName {fieldName = columnName column, fieldNameEntity = entityAliasText}
--       )

fieldSourceProjections :: FieldSource -> Projection
fieldSourceProjections =
  \case
    ExpressionFieldSource aliasedExpression ->
      ExpressionProjection aliasedExpression
    JoinFieldSource cardinality aliasedJoin ->
      ExpressionProjection
        ( aliasedJoin
            { aliasedThing =
                -- Basically a cast, to ensure that SQL Server won't
                -- double-encode the JSON but will "pass it through"
                -- untouched.
                safeJsonQueryExpression
                  cardinality
                  ( ColumnExpression
                      ( joinAliasToField
                          (joinJoinAlias (aliasedThing aliasedJoin))
                      )
                  )
            }
        )

joinAliasToField :: JoinAlias -> FieldName
joinAliasToField JoinAlias {..} =
  FieldName
    { fieldNameEntity = joinAliasEntity,
      fieldName = fromMaybe (error "TODO: Eliminate this case. joinAliasToField") joinAliasField
    }

fieldSourceJoin :: FieldSource -> Maybe Join
fieldSourceJoin =
  \case
    JoinFieldSource _ aliasedJoin -> pure (aliasedThing aliasedJoin)
    ExpressionFieldSource {} -> Nothing

--------------------------------------------------------------------------------
-- Joins

fromObjectRelationSelectG ::
  Map TableName EntityAlias ->
  IR.ObjectRelationSelectG 'MSSQL Void Expression ->
  ReaderT EntityAlias FromIr Join
fromObjectRelationSelectG existingJoins annRelationSelectG = do
  eitherAliasOrFrom <- lift (lookupTableFrom existingJoins tableFrom)
  let entityAlias :: EntityAlias = either id fromAlias eitherAliasOrFrom
  fieldSources <-
    local
      (const entityAlias)
      (traverse (fromAnnFieldsG mempty IR.LeaveNumbersAlone) fields)
  let selectProjections = map fieldSourceProjections fieldSources
  joinJoinAlias <-
    do
      fieldName <- lift (fromRelName _aarRelationshipName)
      alias <- lift (generateAlias (ObjectRelationTemplate fieldName))
      pure
        JoinAlias
          { joinAliasEntity = alias,
            joinAliasField = pure jsonFieldName
          }
  let selectFor =
        JsonFor ForJson {jsonCardinality = JsonSingleton, jsonRoot = NoRoot}
  filterExpression <- local (const entityAlias) (fromGBoolExp tableFilter)
  case eitherAliasOrFrom of
    Right selectFrom -> do
      foreignKeyConditions <- fromMapping selectFrom mapping
      pure
        Join
          { joinJoinAlias,
            joinSource =
              JoinSelect
                emptySelect
                  { selectOrderBy = Nothing,
                    selectTop = NoTop,
                    selectProjections,
                    selectFrom = Just selectFrom,
                    selectJoins = mapMaybe fieldSourceJoin fieldSources,
                    selectWhere =
                      Where (foreignKeyConditions <> [filterExpression]),
                    selectFor,
                    selectOffset = Nothing
                  }
          }
    Left _entityAlias ->
      pure
        Join
          { joinJoinAlias,
            joinSource =
              JoinReselect
                Reselect
                  { reselectProjections = selectProjections,
                    reselectFor = selectFor,
                    reselectWhere = Where [filterExpression]
                  }
          }
  where
    IR.AnnObjectSelectG
      { _aosFields = fields :: IR.AnnFieldsG 'MSSQL Void Expression,
        _aosTableFrom = tableFrom :: TableName,
        _aosTableFilter = tableFilter :: IR.AnnBoolExp 'MSSQL Expression
      } = annObjectSelectG
    IR.AnnRelationSelectG
      { _aarRelationshipName,
        _aarColumnMapping = mapping :: HashMap ColumnName ColumnName,
        _aarAnnSelect = annObjectSelectG :: IR.AnnObjectSelectG 'MSSQL Void Expression
      } = annRelationSelectG

lookupTableFrom ::
  Map TableName EntityAlias ->
  TableName ->
  FromIr (Either EntityAlias From)
lookupTableFrom existingJoins tableFrom = do
  case M.lookup tableFrom existingJoins of
    Just entityAlias -> pure (Left entityAlias)
    Nothing -> fmap Right (fromQualifiedTable tableFrom)

fromArraySelectG :: IR.ArraySelectG 'MSSQL Void Expression -> ReaderT EntityAlias FromIr Join
fromArraySelectG =
  \case
    IR.ASSimple arrayRelationSelectG ->
      fromArrayRelationSelectG arrayRelationSelectG
    IR.ASAggregate arrayAggregateSelectG ->
      fromArrayAggregateSelectG arrayAggregateSelectG

fromArrayAggregateSelectG ::
  IR.AnnRelationSelectG 'MSSQL (IR.AnnAggregateSelectG 'MSSQL Void Expression) ->
  ReaderT EntityAlias FromIr Join
fromArrayAggregateSelectG annRelationSelectG = do
  fieldName <- lift (fromRelName _aarRelationshipName)
  joinSelect <- do
    lhsEntityAlias <- ask
    -- With this, the foreign key relations are injected automatically
    -- at the right place by fromSelectAggregate.
    lift (fromSelectAggregate (pure (lhsEntityAlias, mapping)) annSelectG)
  alias <- lift (generateAlias (ArrayAggregateTemplate fieldName))
  pure
    Join
      { joinJoinAlias =
          JoinAlias
            { joinAliasEntity = alias,
              joinAliasField = pure jsonFieldName
            },
        joinSource = JoinSelect joinSelect
      }
  where
    IR.AnnRelationSelectG
      { _aarRelationshipName,
        _aarColumnMapping = mapping :: HashMap ColumnName ColumnName,
        _aarAnnSelect = annSelectG
      } = annRelationSelectG

fromArrayRelationSelectG :: IR.ArrayRelationSelectG 'MSSQL Void Expression -> ReaderT EntityAlias FromIr Join
fromArrayRelationSelectG annRelationSelectG = do
  fieldName <- lift (fromRelName _aarRelationshipName)
  sel <- lift (fromSelectRows annSelectG)
  joinSelect <-
    do
      foreignKeyConditions <- selectFromMapping sel mapping
      pure
        sel {selectWhere = Where foreignKeyConditions <> selectWhere sel}
  alias <- lift (generateAlias (ArrayRelationTemplate fieldName))
  pure
    Join
      { joinJoinAlias =
          JoinAlias
            { joinAliasEntity = alias,
              joinAliasField = pure jsonFieldName
            },
        joinSource = JoinSelect joinSelect
      }
  where
    IR.AnnRelationSelectG
      { _aarRelationshipName,
        _aarColumnMapping = mapping :: HashMap ColumnName ColumnName,
        _aarAnnSelect = annSelectG
      } = annRelationSelectG

fromRelName :: IR.RelName -> FromIr Text
fromRelName relName =
  pure (IR.relNameToTxt relName)

-- | The context given by the reader is of the previous/parent
-- "remote" table. The WHERE that we're generating goes in the child,
-- "local" query. The @From@ passed in as argument is the local table.
--
-- We should hope to see e.g. "post.category = category.id" for a
-- local table of post and a remote table of category.
--
-- The left/right columns in @HashMap ColumnName ColumnName@ corresponds
-- to the left/right of @select ... join ...@. Therefore left=remote,
-- right=local in this context.
fromMapping ::
  From ->
  HashMap ColumnName ColumnName ->
  ReaderT EntityAlias FromIr [Expression]
fromMapping localFrom =
  traverse
    ( \(remoteColumn, localColumn) -> do
        localFieldName <- local (const (fromAlias localFrom)) (fromColumn localColumn)
        remoteFieldName <- fromColumn remoteColumn
        pure
          ( OpExpression
              TSQL.EQ'
              (ColumnExpression localFieldName)
              (ColumnExpression remoteFieldName)
          )
    )
    . HM.toList

selectFromMapping ::
  Select ->
  HashMap ColumnName ColumnName ->
  ReaderT EntityAlias FromIr [Expression]
selectFromMapping Select {selectFrom = Nothing} = const (pure [])
selectFromMapping Select {selectFrom = Just from} = fromMapping from

--------------------------------------------------------------------------------
-- Basic SQL expression types

fromOpExpG :: Expression -> IR.OpExpG 'MSSQL Expression -> FromIr Expression
fromOpExpG expression op =
  case op of
    IR.ANISNULL -> pure $ TSQL.IsNullExpression expression
    IR.ANISNOTNULL -> pure $ TSQL.IsNotNullExpression expression
    IR.AEQ False val -> pure $ nullableBoolEquality expression val
    IR.AEQ True val -> pure $ OpExpression TSQL.EQ' expression val
    IR.ANE False val -> pure $ nullableBoolInequality expression val
    IR.ANE True val -> pure $ OpExpression TSQL.NEQ' expression val
    IR.AGT val -> pure $ OpExpression TSQL.GT expression val
    IR.ALT val -> pure $ OpExpression TSQL.LT expression val
    IR.AGTE val -> pure $ OpExpression TSQL.GTE expression val
    IR.ALTE val -> pure $ OpExpression TSQL.LTE expression val
    IR.AIN val -> pure $ OpExpression TSQL.IN expression val
    IR.ANIN val -> pure $ OpExpression TSQL.NIN expression val
    IR.ALIKE val -> pure $ OpExpression TSQL.LIKE expression val
    IR.ANLIKE val -> pure $ OpExpression TSQL.NLIKE expression val
    IR.ABackendSpecific o -> case o of
      ASTContains val -> pure $ TSQL.STOpExpression TSQL.STContains expression val
      ASTCrosses val -> pure $ TSQL.STOpExpression TSQL.STCrosses expression val
      ASTEquals val -> pure $ TSQL.STOpExpression TSQL.STEquals expression val
      ASTIntersects val -> pure $ TSQL.STOpExpression TSQL.STIntersects expression val
      ASTOverlaps val -> pure $ TSQL.STOpExpression TSQL.STOverlaps expression val
      ASTTouches val -> pure $ TSQL.STOpExpression TSQL.STTouches expression val
      ASTWithin val -> pure $ TSQL.STOpExpression TSQL.STWithin expression val
    -- As of March 2021, only geometry/geography casts are supported
    IR.ACast _casts -> refute (pure (UnsupportedOpExpG op)) -- mkCastsExp casts

    -- We do not yet support column names in permissions
    IR.CEQ _rhsCol -> refute (pure (UnsupportedOpExpG op)) -- S.BECompare S.SEQ lhs $ mkQCol rhsCol
    IR.CNE _rhsCol -> refute (pure (UnsupportedOpExpG op)) -- S.BECompare S.SNE lhs $ mkQCol rhsCol
    IR.CGT _rhsCol -> refute (pure (UnsupportedOpExpG op)) -- S.BECompare S.SGT lhs $ mkQCol rhsCol
    IR.CLT _rhsCol -> refute (pure (UnsupportedOpExpG op)) -- S.BECompare S.SLT lhs $ mkQCol rhsCol
    IR.CGTE _rhsCol -> refute (pure (UnsupportedOpExpG op)) -- S.BECompare S.SGTE lhs $ mkQCol rhsCol
    IR.CLTE _rhsCol -> refute (pure (UnsupportedOpExpG op)) -- S.BECompare S.SLTE lhs $ mkQCol rhsCol

nullableBoolEquality :: Expression -> Expression -> Expression
nullableBoolEquality x y =
  OrExpression
    [ OpExpression TSQL.EQ' x y,
      AndExpression [IsNullExpression x, IsNullExpression y]
    ]

nullableBoolInequality :: Expression -> Expression -> Expression
nullableBoolInequality x y =
  OrExpression
    [ OpExpression TSQL.NEQ' x y,
      AndExpression [IsNotNullExpression x, IsNullExpression y]
    ]

-- | Translate a 'GBoolExp' of a 'AnnBoolExpFld', within an 'EntityAlias' context.
--
-- It is mutually recursive with 'fromAnnBoolExpFld' and 'fromGExists'.
fromGBoolExp ::
  IR.GBoolExp 'MSSQL (IR.AnnBoolExpFld 'MSSQL Expression) ->
  ReaderT EntityAlias FromIr Expression
fromGBoolExp =
  \case
    IR.BoolAnd expressions ->
      fmap AndExpression (traverse fromGBoolExp expressions)
    IR.BoolOr expressions ->
      fmap OrExpression (traverse fromGBoolExp expressions)
    IR.BoolNot expression ->
      fmap NotExpression (fromGBoolExp expression)
    IR.BoolExists gExists ->
      fromGExists gExists
    IR.BoolFld expression ->
      fromAnnBoolExpFld expression
  where
    fromGExists :: IR.GExists 'MSSQL (IR.AnnBoolExpFld 'MSSQL Expression) -> ReaderT EntityAlias FromIr Expression
    fromGExists IR.GExists {_geTable, _geWhere} = do
      selectFrom <- lift (fromQualifiedTable _geTable)
      whereExpression <-
        local (const (fromAlias selectFrom)) (fromGBoolExp _geWhere)
      pure $
        ExistsExpression $
          emptySelect
            { selectOrderBy = Nothing,
              selectProjections =
                [ ExpressionProjection
                    ( Aliased
                        { aliasedThing = trueExpression,
                          aliasedAlias = existsFieldName
                        }
                    )
                ],
              selectFrom = Just selectFrom,
              selectJoins = mempty,
              selectWhere = Where [whereExpression],
              selectTop = NoTop,
              selectFor = NoFor,
              selectOffset = Nothing
            }

trueExpression :: Expression
trueExpression = ValueExpression (ODBC.BoolValue True)

-- | A version of @JSON_QUERY(..)@ that returns a proper json literal, rather
-- than SQL null, which does not compose properly with @FOR JSON@ clauses.
safeJsonQueryExpression :: JsonCardinality -> Expression -> Expression
safeJsonQueryExpression expectedType jsonQuery =
  FunctionApplicationExpression (FunExpISNULL (JsonQueryExpression jsonQuery) jsonTypeExpression)
  where
    jsonTypeExpression = case expectedType of
      JsonSingleton -> nullExpression
      JsonArray -> emptyArrayExpression

nullExpression :: Expression
nullExpression = ValueExpression $ ODBC.TextValue "null"

emptyArrayExpression :: Expression
emptyArrayExpression = ValueExpression $ ODBC.TextValue "[]"

--------------------------------------------------------------------------------
-- Constants

jsonFieldName :: Text
jsonFieldName = "json"

aggSubselectName :: Text
aggSubselectName = "agg_sub"

existsFieldName :: Text
existsFieldName = "exists_placeholder"

data UnfurledJoin = UnfurledJoin
  { unfurledJoin :: Join,
    -- | Recorded if we joined onto an object relation.
    unfurledObjectTableAlias :: Maybe (TableName, EntityAlias)
  }
  deriving (Show)

fromAnnotatedOrderByItemG ::
  IR.AnnotatedOrderByItemG 'MSSQL Expression ->
  WriterT (Seq UnfurledJoin) (ReaderT EntityAlias FromIr) OrderBy
fromAnnotatedOrderByItemG IR.OrderByItemG {obiType, obiColumn = obiColumn, obiNulls} = do
  (orderByFieldName, orderByType) <- unfurlAnnotatedOrderByElement obiColumn
  let orderByNullsOrder = fromMaybe NullsAnyOrder obiNulls
      orderByOrder = fromMaybe AscOrder obiType
  pure OrderBy {..}

-- | Unfurl the nested set of object relations (tell'd in the writer)
-- that are terminated by field name (IR.AOCColumn and
-- IR.AOCArrayAggregation).
unfurlAnnotatedOrderByElement ::
  IR.AnnotatedOrderByElement 'MSSQL Expression ->
  WriterT (Seq UnfurledJoin) (ReaderT EntityAlias FromIr) (FieldName, Maybe TSQL.ScalarType)
unfurlAnnotatedOrderByElement =
  \case
    IR.AOCColumn columnInfo -> do
      fieldName <- lift (fromColumnInfo columnInfo)
      pure
        ( fieldName,
          case IR.ciType columnInfo of
            IR.ColumnScalar t -> Just t
            -- Above: It is of interest to us whether the type is
            -- text/ntext/image. See ToQuery for more explanation.
            _ -> Nothing
        )
    IR.AOCObjectRelation IR.RelInfo {riMapping = mapping, riRTable = table} annBoolExp annOrderByElementG -> do
      selectFrom <- lift (lift (fromQualifiedTable table))
      joinAliasEntity <-
        lift (lift (generateAlias (ForOrderAlias (tableNameText table))))
      foreignKeyConditions <- lift (fromMapping selectFrom mapping)
      -- TODO: Because these object relations are re-used by regular
      -- object mapping queries, this WHERE may be unnecessarily
      -- restrictive. But I actually don't know from where such an
      -- expression arises in the source GraphQL syntax.
      --
      -- Worst case scenario, we could put the WHERE in the key of the
      -- Map in 'argsExistingJoins'. That would guarantee only equal
      -- selects are re-used.
      whereExpression <-
        lift (local (const (fromAlias selectFrom)) (fromGBoolExp annBoolExp))
      tell
        ( pure
            UnfurledJoin
              { unfurledJoin =
                  Join
                    { joinSource =
                        JoinSelect
                          emptySelect
                            { selectTop = NoTop,
                              selectProjections = [StarProjection],
                              selectFrom = Just selectFrom,
                              selectJoins = [],
                              selectWhere =
                                Where (foreignKeyConditions <> [whereExpression]),
                              selectFor = NoFor,
                              selectOrderBy = Nothing,
                              selectOffset = Nothing
                            },
                      joinJoinAlias =
                        JoinAlias {joinAliasEntity, joinAliasField = Nothing}
                    },
                unfurledObjectTableAlias = Just (table, EntityAlias joinAliasEntity)
              }
        )
      local
        (const (EntityAlias joinAliasEntity))
        (unfurlAnnotatedOrderByElement annOrderByElementG)
    IR.AOCArrayAggregation IR.RelInfo {riMapping = mapping, riRTable = tableName} annBoolExp annAggregateOrderBy -> do
      selectFrom <- lift (lift (fromQualifiedTable tableName))
      let alias = aggFieldName
      joinAliasEntity <-
        lift (lift (generateAlias (ForOrderAlias (tableNameText tableName))))
      foreignKeyConditions <- lift (fromMapping selectFrom mapping)
      whereExpression <-
        lift (local (const (fromAlias selectFrom)) (fromGBoolExp annBoolExp))
      aggregate <-
        lift
          ( local
              (const (fromAlias selectFrom))
              ( case annAggregateOrderBy of
                  IR.AAOCount -> pure (CountAggregate StarCountable)
                  IR.AAOOp text columnInfo -> do
                    fieldName <- fromColumnInfo columnInfo
                    pure (OpAggregate text (pure (ColumnExpression fieldName)))
              )
          )
      tell
        ( pure
            ( UnfurledJoin
                { unfurledJoin =
                    Join
                      { joinSource =
                          JoinSelect
                            emptySelect
                              { selectTop = NoTop,
                                selectProjections =
                                  [ AggregateProjection
                                      Aliased
                                        { aliasedThing = aggregate,
                                          aliasedAlias = alias
                                        }
                                  ],
                                selectFrom = Just selectFrom,
                                selectJoins = [],
                                selectWhere =
                                  Where
                                    (foreignKeyConditions <> [whereExpression]),
                                selectFor = NoFor,
                                selectOrderBy = Nothing,
                                selectOffset = Nothing
                              },
                        joinJoinAlias =
                          JoinAlias {joinAliasEntity, joinAliasField = Nothing}
                      },
                  unfurledObjectTableAlias = Nothing
                }
            )
        )
      pure
        ( FieldName {fieldNameEntity = joinAliasEntity, fieldName = alias},
          Nothing
        )

tableNameText :: TableName -> Text
tableNameText (TableName {tableName}) = tableName

fromColumnInfo :: IR.ColumnInfo 'MSSQL -> ReaderT EntityAlias FromIr FieldName
fromColumnInfo IR.ColumnInfo {ciColumn = column} =
  columnNameToFieldName column <$> ask

aggFieldName :: Text
aggFieldName = "agg"