1{-# language DuplicateRecordFields #-}
20 -- *** Deriving-via helpers
28 -- *** @TypeInformation@
31 , parseTypeInformation
33 -- ** The @DBType@ hierarchy
39 -- * Tables and higher-kinded tables
50 , EqTable, (==:), (/=:)
51 , OrdTable, ascTable, descTable
58 , maybeTable, ($?), nothingTable, justTable
59 , isNothingTable, isJustTable
67 , eitherTable, leftTable, rightTable
68 , isLeftTable, isRightTable
72 , bitraverseEitherTable
76 , theseTable, thisTable, thatTable, thoseTable
77 , isThisTable, isThatTable, isThoseTable
78 , hasHereTable, hasThereTable
79 , justHereTable, justThereTable
81 , keepHereTable, loseHereTable
82 , keepThereTable, loseThereTable
83 , keepThisTable, loseThisTable
84 , keepThatTable, loseThatTable
85 , keepThoseTable, loseThoseTable
87 , bitraverseTheseTable
119 -- ** Boolean operations
124 , (==.), (/=.), (==?), (/=?)
130 , (<.), (<=.), (>.), (>=.)
131 , (<?), (<=?), (>?), (>=?)
132 , leastExpr, greatestExpr
158 -- ** @LIMIT@/@OFFSET@
190 , DBString, stringAgg
209 -- * Running statements
233 -- These need organizing, but are reachable from Rel8's documentation so we
234 -- do need to export and document them.
248import Rel8.Expr.Aggregate
251import Rel8.Expr.Function
254import Rel8.Expr.Order
255import Rel8.Expr.Serialize
258import Rel8.Query.Aggregate
259import Rel8.Query.Distinct
260import Rel8.Query.Each
261import Rel8.Query.Either
262import Rel8.Query.Exists
263import Rel8.Query.Filter
264import Rel8.Query.Limit
265import Rel8.Query.List
266import Rel8.Query.Maybe
267import Rel8.Query.Null
268import Rel8.Query.Order
269import Rel8.Query.SQL (showQuery)
271import Rel8.Query.These
272import Rel8.Query.Values
273import Rel8.Schema.Column
274import Rel8.Schema.Context.Label
275import Rel8.Schema.Generic
276import Rel8.Schema.HTable
277import Rel8.Schema.Name
278import Rel8.Schema.Nullability
279import Rel8.Schema.Table
280import Rel8.Statement.Delete
281import Rel8.Statement.Insert
282import Rel8.Statement.Returning
283import Rel8.Statement.Select
284import Rel8.Statement.Update
286import Rel8.Table.Aggregate
287import Rel8.Table.Alternative
288import Rel8.Table.Bool
289import Rel8.Table.Either
291import Rel8.Table.Insert
292import Rel8.Table.List
293import Rel8.Table.Maybe
294import Rel8.Table.Name
295import Rel8.Table.NonEmpty
297import Rel8.Table.Order
298import Rel8.Table.Serialize
299import Rel8.Table.These
302import Rel8.Type.Information
303import Rel8.Type.JSONEncoded
304import Rel8.Type.JSONBEncoded
305import Rel8.Type.Monoid
308import Rel8.Type.ReadShow
309import Rel8.Type.Semigroup
310import Rel8.Type.String
312import Rel8.Expr.Opaleye (unsafeCastExpr)
317-- In this section, we'll take a look at using Rel8 to work with a small
318-- database for Haskell packages. We'll take a look at idiomatic usage of Rel8,
319-- mapping tables to Haskell, and then look at writing some simple queries.
321-- The documentation in this library uses the following language extensions:
323-- >>> :set -XBlockArguments
324-- >>> :set -XDeriveAnyClass
325-- >>> :set -XDeriveGeneric
326-- >>> :set -XDerivingStrategies
327-- >>> :set -XDerivingVia
328-- >>> :set -XDuplicateRecordFields
329-- >>> :set -XGeneralizedNewtypeDeriving
330-- >>> :set -XOverloadedStrings
331-- >>> :set -XStandaloneDeriving
332-- >>> :set -XTypeApplications
333-- >>> :set -XTypeFamilies
335-- Before we start writing any Haskell, let's take a look at the schema we'll
340-- > Column │ Type │ Nullable Column │ Type │ Nullable
341-- > ═══════════╪═════════╪══════════ ═══════════╪═════════╪══════════
342-- > author_id │ integer │ not null author_id │ integer │ not null
343-- > name │ text │ not null name │ text │ not null
346-- Our schema consists of two tables - @author@ and @project@. A @project@
347-- always has an @author@, but not all @author@s have projects. Each @author@
348-- has a name and (maybe) an associated website, and each project has a name.
350-- Now that we've seen our schema, we can begin writing a mapping in Rel8. The
351-- idiomatic way to map a table is to use a record that is parameterised what
352-- Rel8 calls an /interpretation functor/, and to define each field with
353-- 'Column'. For this type to be usable with Rel8 we need it to be an instance
354-- of 'HigherKindedTable', which can be derived with a combination of
355-- @DeriveAnyClass@ and @DeriveGeneric@.
357-- Following these steps for @author@, we have:
359-- > data Author f = Author
360-- > { authorId :: Column f Int64
361-- > , name :: Column f Text
362-- > , url :: Column f (Maybe Text)
363-- > } deriving (Generic, HigherKindedTable)
365-- This is a perfectly reasonable definition, but cautious readers might notice
366-- a problem - in particular, with the type of the @authorId@ field. While
367-- @Int64@ is correct, it's not the best type. If we had other identifier types
368-- in our project, it would be too easy to accidentally mix them up and create
369-- nonsensical joins. As Haskell programmers, we often solve this problem by
370-- creating @newtype@ wrappers, and we can also use this technique with Rel8:
373-- newtype AuthorId = AuthorId { toInt64 :: Int64 }
374-- deriving newtype (DBEq, DBType, Eq, Show)
377-- Now we can write our final schema mapping. First, the @author@ table:
380-- data Author f = Author
381-- { authorId :: Column f AuthorId
382-- , authorName :: Column f Text
383-- , authorUrl :: Column f (Maybe Text)
385-- deriving stock Generic
386-- deriving anyclass HigherKindedTable
389-- And similarly, the @project@ table:
392-- data Project f = Project
393-- { projectAuthorId :: Column f AuthorId
394-- , projectName :: Column f Text
396-- deriving stock Generic
397-- deriving anyclass HigherKindedTable
400-- To show query results in this documentation, we'll also need @Show@
401-- instances: Unfortunately these definitions look a bit scary, but they are
402-- essentially just @deriving (Show)@:
404-- >>> deriving stock instance f ~ Identity => Show (Author f)
405-- >>> deriving stock instance f ~ Identity => Show (Project f)
407-- These data types describe the structural mapping of the tables, but we also
408-- need to specify a 'TableSchema' for each table. A @TableSchema@ contains the
409-- name of the table and the name of all columns in the table, which will
410-- ultimately allow us to @SELECT@ and @INSERT@ rows for these tables.
412-- To define a @TableSchema@, we just need to fill construct appropriate
413-- @TableSchema@ values. When it comes to the @tableColumns@ field, we
414-- construct values of our data types above, and set each field to the name of
415-- the column that it maps to:
417-- First, @authorSchema@ describes the column names of the @author@ table when
418-- associated with the @Author@ type:
421-- authorSchema :: TableSchema (Author ColumnSchema)
422-- authorSchema = TableSchema
423-- { tableName = "author"
424-- , tableSchema = Nothing
425-- , tableColumns = Author
426-- { authorId = "author_id"
427-- , authorName = "name"
428-- , authorUrl = "url"
433-- And likewise for @project@ and @Project@:
436-- projectSchema :: TableSchema (Project ColumnSchema)
437-- projectSchema = TableSchema
438-- { tableName = "project"
439-- , tableSchema = Nothing
440-- , tableColumns = Project
441-- { projectAuthorId = "author_id"
442-- , projectName = "name"
447-- Aside: you might be wondering why this information isn't in the definitions
448-- of @Author@ and @Project@ above. Rel8 decouples @TableSchema@ from the data
449-- types themselves, as not all tables you define will necessarily have a
450-- schema. For example, Rel8 allows you to define helper types to simplify the
451-- types of queries - these tables only exist at query time, but there is no
452-- corresponding base table. We'll see more on this idea later!
454-- With these table definitions, we can now start writing some queries! To
455-- actually run a query, we'll need a database connection. Rel8 uses the
456-- @hasql@ library, and for this documentation we'll get the connection string
457-- from the @$TEST_DATABASE_URL@ environment variable.
459-- >>> connectionString <- System.Environment.getEnv "TEST_DATABASE_URL"
460-- >>> Right c <- Hasql.Connection.acquire (Data.ByteString.Char8.pack connectionString)
461-- >>> Control.Monad.void $ Hasql.Session.run (Hasql.Session.sql "BEGIN") c
465-- First, we'll take a look at @SELECT@ statements - usually the bulk of most
466-- database heavy applications.
468-- In Rel8, @SELECT@ statements are built using the 'Query' monad. You can
469-- think of this monad like the ordinary @[]@ (List) monad - but this isn't
470-- required knowledge.
472-- To start, we'll look at one of the simplest queries possible - a basic
473-- @SELECT * FROM@ statement. To select all rows from a table, we use 'each',
474-- and supply a 'TableSchema'. So to select all @project@ rows, we can write:
476-- >>> :t each projectSchema
477-- each projectSchema :: Query (Project Expr)
479-- Notice that @each@ gives us a @Query@ that yields @Project Expr@ rows. To
480-- see what this means, let's have a look at a single field of a
483-- >>> let aProjectExpr = undefined :: Project Expr
484-- >>> :t projectAuthorId aProjectExpr
485-- projectAuthorId aProjectExpr :: Expr AuthorId
487-- Recall we defined @projectAuthorId@ as @Column f AuthorId@. Now we have @f@
488-- is @Expr@, and @Column Expr AuthorId@ reduces to @Expr AuthorId@. We'll see
489-- more about @Expr@ soon, but you can think of @Expr a@ as "SQL expressions of
492-- To execute this @Query@, we pass it to 'select':
494-- >>> :t select c (each projectSchema)
495-- select c (each projectSchema) :: MonadIO m => m [Project Identity]
497-- When we @select@ things containing @Expr@s, Rel8 builds a new response table
498-- with the @Identity@ interpretation. This means you'll get back plain Haskell
499-- values. Studying @projectAuthorId@ again, we have:
501-- >>> let aProjectIdentity = undefined :: Project Identity
502-- >>> :t projectAuthorId aProjectIdentity
503-- projectAuthorId aProjectIdentity :: AuthorId
505-- Here @Column Identity AuthorId@ reduces to just @AuthorId@, with no
506-- wrappping type at all.
508-- Putting this all together, we can run our first query:
510-- >>> select c (each projectSchema) >>= mapM_ print
511-- Project {projectAuthorId = 1, projectName = "rel8"}
512-- Project {projectAuthorId = 2, projectName = "aeson"}
513-- Project {projectAuthorId = 2, projectName = "text"}
517-- We now know that 'each' is the equivalent of a @SELECT *@ query, but
518-- sometimes we're only interested in a subset of the columns of a table. To
519-- restrict the returned columns, we can specify a projection by using 'Query's
520-- @Functor@ instance:
522-- >>> select c $ projectName <$> each projectSchema
523-- ["rel8","aeson","text"]
527-- Another common operation in relational databases is to take the @JOIN@ of
528-- multiple tables. Rel8 doesn't have a specific join operation, but we can
529-- recover the functionality of a join by selecting all rows of two tables, and
530-- then using 'where_' to filter them.
532-- To see how this works, first let's look at taking the product of two tables.
533-- We can do this by simply calling 'each' twice, and then returning a tuple of
537-- mapM_ print =<< select c do
538-- author <- each authorSchema
539-- project <- each projectSchema
540-- return (projectName project, authorName author)
543-- ("rel8","Bryan O'Sullivan")
544-- ("rel8","Emily Pillmore")
546-- ("aeson","Bryan O'Sullivan")
547-- ("aeson","Emily Pillmore")
549-- ("text","Bryan O'Sullivan")
550-- ("text","Emily Pillmore")
552-- This isn't quite right, though, as we have ended up pairing up the wrong
553-- projects and authors. To fix this, we can use 'where_' to restrict the
554-- returned rows. We could write:
557-- > author <- each authorSchema
558-- > project <- each projectSchema
559-- > where_ $ projectAuthorId project ==. authorId author
560-- > return (project, author)
562-- but doing this every time you need a join can obscure the meaning of the
563-- query you're writing. A good practice is to introduce specialised functions
564-- for the particular joins in your database. In our case, this would be:
567-- projectsForAuthor :: Author Expr -> Query (Project Expr)
568-- projectsForAuthor a = each projectSchema >>= filter \p ->
569-- projectAuthorId p ==. authorId a
572-- Our final query is then:
575-- mapM_ print =<< select c do
576-- author <- each authorSchema
577-- project <- projectsForAuthor author
578-- return (projectName project, authorName author)
581-- ("aeson","Bryan O'Sullivan")
582-- ("text","Bryan O'Sullivan")
586-- Rel8 is also capable of performing @LEFT JOIN@s. To perform @LEFT JOIN@s, we
587-- follow the same approach as before, but use the 'optional' query transformer
588-- to allow for the possibility of the join to fail.
590-- In our test database, we can see that there's another author who we haven't
593-- >>> select c $ authorName <$> each authorSchema
594-- ["Ollie","Bryan O'Sullivan","Emily Pillmore"]
596-- Emily wasn't returned in our earlier query because - in our database - she
597-- doesn't have any registered projects. We can account for this partiality in
598-- our original query by wrapping the @projectsForAuthor@ call with 'optional':
601-- mapM_ print =<< select c do
602-- author <- each authorSchema
603-- mproject <- optional $ projectsForAuthor author
604-- return (authorName author, projectName <$> mproject)
606-- ("Ollie",Just "rel8")
607-- ("Bryan O'Sullivan",Just "aeson")
608-- ("Bryan O'Sullivan",Just "text")
609-- ("Emily Pillmore",Nothing)
613-- Aggregations are operations like @sum@ and @count@ - operations that reduce
614-- multiple rows to single values. To perform aggregations in Rel8, we can use
615-- the 'aggregate' function, which takes a 'Query' of aggregated expressions,
616-- runs the aggregation, and returns aggregated rows.
618-- To start, let's look at a simple aggregation that tells us how many projects
623-- Rel8 is also capable of aggregating multiple rows into a single row by
624-- concatenating all rows as a list. This aggregation allows us to break free
625-- of the row-orientated nature of SQL and write queries that return tree-like
626-- structures. Earlier we saw an example of returning authors with their
627-- projects, but the query didn't do a great job of describing the one-to-many
628-- relationship between authors and their projects.
630-- Let's look again at a query that returns authors and their projects, and
631-- focus on the /type/ of that query.
634-- projectsForAuthor a = each projectSchema >>= filter \p ->
635-- projectAuthorId p ==. authorId a
639-- let authorsAndProjects = do
640-- author <- each authorSchema
641-- project <- projectsForAuthor author
642-- return (author, project)
646-- >>> :t select c authorsAndProjects
647-- select c authorsAndProjects
648-- :: MonadIO m => m [(Author Identity, Project Identity)]
650-- Our query gives us a single list of pairs of authors and projects. However,
651-- with our domain knowledge of the schema, this isn't a great type - what we'd
652-- rather have is a list of pairs of authors and /lists/ of projects. That is,
655-- > [(Author Identity, [Project Identity])]
657-- This would be a much better type! Rel8 can produce a query with this type by
658-- simply wrapping the call to @projectsForAuthor@ with either 'some' or
659-- 'many'. Here we'll use 'many', which allows for the possibility of an author
660-- to have no projects:
663-- mapM_ print =<< select c do
664-- author <- each authorSchema
665-- projectNames <- many $ projectName <$> projectsForAuthor author
666-- return (authorName author, projectNames)
669-- ("Bryan O'Sullivan",["aeson","text"])
670-- ("Emily Pillmore",[])
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