Implement 'rename column' migrations (#52)

Add support for **rename column** migrations. A rename column migration
looks like:

```json
{
  "name": "13_rename_column",
  "operations": [
    {
      "rename_column": {
        "table": "employees",
        "from": "role",
        "to": "jobTitle"
      }
    }
  ]
}
```

* On `Start`, the view in the new version schema aliases the renamed
column to its new name. The column in the underlying table is not
renamed.
* `Rollback` is a no-op.
* `Complete` renames the column in the underlying table.

examples/12_create_employees_table.json

@@ -0,0 +1,21 @@
+{
+  "name": "12_create_employees_table",
+  "operations": [
+    {
+      "create_table": {
+        "name": "employees",
+        "columns": [
+          {
+            "name": "id",
+            "type": "serial",
+            "pk": true
+          },
+          {
+            "name": "role",
+            "type": "varchar(255)"
+          }
+        ]
+      }
+    }
+  ]
+}

examples/13_rename_column.json

@@ -0,0 +1,12 @@
+{
+  "name": "13_rename_column",
+  "operations": [
+    {
+      "rename_column": {
+        "table": "employees",
+        "from": "role",
+        "to": "jobTitle"
+      }
+    }
+  ]
+}

pkg/migrations/op_common.go

@@ -11,13 +11,14 @@ import (
 type OpName string
 
 const (
-	OpNameCreateTable OpName = "create_table"
-	OpNameRenameTable OpName = "rename_table"
-	OpNameDropTable   OpName = "drop_table"
-	OpNameAddColumn   OpName = "add_column"
-	OpNameDropColumn  OpName = "drop_column"
-	OpNameCreateIndex OpName = "create_index"
-	OpNameDropIndex   OpName = "drop_index"
+	OpNameCreateTable  OpName = "create_table"
+	OpNameRenameTable  OpName = "rename_table"
+	OpNameDropTable    OpName = "drop_table"
+	OpNameAddColumn    OpName = "add_column"
+	OpNameDropColumn   OpName = "drop_column"
+	OpNameCreateIndex  OpName = "create_index"
+	OpNameDropIndex    OpName = "drop_index"
+	OpNameRenameColumn OpName = "rename_column"
 )
 
 func TemporaryName(name string) string {
@@ -88,6 +89,9 @@ func (v *Operations) UnmarshalJSON(data []byte) error {
 		case OpNameDropColumn:
 			item = &OpDropColumn{}
 
+		case OpNameRenameColumn:
+			item = &OpRenameColumn{}
+
 		case OpNameCreateIndex:
 			item = &OpCreateIndex{}
 
@@ -141,6 +145,9 @@ func (v Operations) MarshalJSON() ([]byte, error) {
 		case *OpDropColumn:
 			opName = OpNameDropColumn
 
+		case *OpRenameColumn:
+			opName = OpNameRenameColumn
+
 		case *OpCreateIndex:
 			opName = OpNameCreateIndex
 

pkg/migrations/op_rename_column.go

@@ -0,0 +1,56 @@
+package migrations
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"github.com/lib/pq"
+
+	"pg-roll/pkg/schema"
+)
+
+type OpRenameColumn struct {
+	Table string `json:"table"`
+	From  string `json:"from"`
+	To    string `json:"to"`
+}
+
+var _ Operation = (*OpRenameColumn)(nil)
+
+func (o *OpRenameColumn) Start(ctx context.Context, conn *sql.DB, schemaName string, stateSchema string, s *schema.Schema) error {
+	table := s.GetTable(o.Table)
+	table.RenameColumn(o.From, o.To)
+	return nil
+}
+
+func (o *OpRenameColumn) Complete(ctx context.Context, conn *sql.DB) error {
+	// rename the column in the underlying table
+	_, err := conn.ExecContext(ctx, fmt.Sprintf("ALTER TABLE %s RENAME COLUMN %s TO %s",
+		pq.QuoteIdentifier(o.Table),
+		pq.QuoteIdentifier(o.From),
+		pq.QuoteIdentifier(o.To)))
+	return err
+}
+
+func (o *OpRenameColumn) Rollback(ctx context.Context, conn *sql.DB) error {
+	// no-op
+	return nil
+}
+
+func (o *OpRenameColumn) Validate(ctx context.Context, s *schema.Schema) error {
+	table := s.GetTable(o.Table)
+	if table == nil {
+		return TableDoesNotExistError{Name: o.Table}
+	}
+
+	if table.GetColumn(o.From) == nil {
+		return ColumnDoesNotExistError{Table: o.Table, Name: o.From}
+	}
+
+	if table.GetColumn(o.To) != nil {
+		return ColumnAlreadyExistsError{Table: o.Table, Name: o.From}
+	}
+
+	return nil
+}

pkg/migrations/op_rename_column_test.go

@@ -0,0 +1,81 @@
+package migrations_test
+
+import (
+	"database/sql"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"pg-roll/pkg/migrations"
+)
+
+func TestRenameColumn(t *testing.T) {
+	t.Parallel()
+
+	ExecuteTests(t, TestCases{{
+		name: "rename column",
+		migrations: []migrations.Migration{
+			{
+				Name: "01_add_table",
+				Operations: migrations.Operations{
+					&migrations.OpCreateTable{
+						Name: "users",
+						Columns: []migrations.Column{
+							{
+								Name:       "id",
+								Type:       "serial",
+								PrimaryKey: true,
+							},
+							{
+								Name:     "username",
+								Type:     "varchar(255)",
+								Nullable: false,
+							},
+						},
+					},
+				},
+			},
+			{
+				Name: "02_rename_column",
+				Operations: migrations.Operations{
+					&migrations.OpRenameColumn{
+						Table: "users",
+						From:  "username",
+						To:    "name",
+					},
+				},
+			},
+		},
+		afterStart: func(t *testing.T, db *sql.DB) {
+			// The column in the underlying table has not been renamed.
+			ColumnMustExist(t, db, "public", "users", "username")
+
+			// Insertions to the new column name in the new version schema should work.
+			MustInsert(t, db, "public", "02_rename_column", "users", map[string]string{"name": "alice"})
+
+			// Insertions to the old column name in the old version schema should work.
+			MustInsert(t, db, "public", "01_add_table", "users", map[string]string{"username": "bob"})
+
+			// Data can be read from the view in the new version schema.
+			rows := MustSelect(t, db, "public", "02_rename_column", "users")
+			assert.Equal(t, []map[string]any{
+				{"id": 1, "name": "alice"},
+				{"id": 2, "name": "bob"},
+			}, rows)
+		},
+		afterRollback: func(t *testing.T, db *sql.DB) {
+			// no-op
+		},
+		afterComplete: func(t *testing.T, db *sql.DB) {
+			// The column in the underlying table has been renamed.
+			ColumnMustExist(t, db, "public", "users", "name")
+
+			// Data can be read from the view in the new version schema.
+			rows := MustSelect(t, db, "public", "02_rename_column", "users")
+			assert.Equal(t, []map[string]any{
+				{"id": 1, "name": "alice"},
+				{"id": 2, "name": "bob"},
+			}, rows)
+		},
+	}})
+}

pkg/schema/schema.go

@@ -114,3 +114,8 @@ func (t *Table) AddColumn(name string, c Column) {
 func (t *Table) RemoveColumn(column string) {
 	delete(t.Columns, column)
 }
+
+func (t *Table) RenameColumn(from, to string) {
+	t.Columns[to] = t.Columns[from]
+	delete(t.Columns, from)
+}