Preserve foreign key constraints when changing a column's type (#232)

Preserve any foreign key constraints defined on columns when changing a
column's type.

Uses the `Duplicator` type added in #230 in the 'change type' operation
to ensure that FK constraints are preserved when the column is
duplicated for backfilling.

#230 did the same thing for
preserving FK constraints when a column has a `NOT NULL` constraint
added.

Part of #227

pkg/migrations/duplicate.go

@@ -14,22 +14,29 @@ import (
 )
 
 type Duplicator struct {
-	conn   *sql.DB
-	table  *schema.Table
-	column *schema.Column
-	asName string
+	conn     *sql.DB
+	table    *schema.Table
+	column   *schema.Column
+	asName   string
+	withType string
 }
 
 // NewColumnDuplicator creates a new Duplicator for a column.
 func NewColumnDuplicator(conn *sql.DB, table *schema.Table, column *schema.Column) *Duplicator {
 	return &Duplicator{
-		conn:   conn,
-		table:  table,
-		column: column,
-		asName: TemporaryName(column.Name),
+		conn:     conn,
+		table:    table,
+		column:   column,
+		asName:   TemporaryName(column.Name),
+		withType: column.Type,
 	}
 }
 
+func (d *Duplicator) WithType(t string) *Duplicator {
+	d.withType = t
+	return d
+}
+
 // Duplicate creates a new column with the same type and foreign key
 // constraints as the original column.
 func (d *Duplicator) Duplicate(ctx context.Context) error {
@@ -41,7 +48,7 @@ func (d *Duplicator) Duplicate(ctx context.Context) error {
 	sql := fmt.Sprintf(cAlterTableSQL,
 		pq.QuoteIdentifier(d.table.Name),
 		pq.QuoteIdentifier(d.asName),
-		d.column.Type)
+		d.withType)
 
 	for _, fk := range d.table.ForeignKeys {
 		if slices.Contains(fk.Columns, d.column.Name) {

pkg/migrations/op_change_type.go

@@ -26,7 +26,8 @@ func (o *OpChangeType) Start(ctx context.Context, conn *sql.DB, stateSchema stri
 	column := table.GetColumn(o.Column)
 
 	// Create a copy of the column on the underlying table.
-	if err := duplicateColumnForTypeChange(ctx, conn, table, *column, o.Type); err != nil {
+	d := NewColumnDuplicator(conn, table, column).WithType(o.Type)
+	if err := d.Duplicate(ctx); err != nil {
 		return fmt.Errorf("failed to duplicate column: %w", err)
 	}
 
@@ -99,12 +100,13 @@ func (o *OpChangeType) Complete(ctx context.Context, conn *sql.DB, s *schema.Sch
 	}
 
 	// Rename the new column to the old column name
-	_, err = conn.ExecContext(ctx, fmt.Sprintf("ALTER TABLE IF EXISTS %s RENAME COLUMN %s TO %s",
-		pq.QuoteIdentifier(o.Table),
-		pq.QuoteIdentifier(TemporaryName(o.Column)),
-		pq.QuoteIdentifier(o.Column)))
+	table := s.GetTable(o.Table)
+	column := table.GetColumn(o.Column)
+	if err := RenameDuplicatedColumn(ctx, conn, table, column); err != nil {
+		return err
+	}
 
-	return err
+	return nil
 }
 
 func (o *OpChangeType) Rollback(ctx context.Context, conn *sql.DB) error {
@@ -143,15 +145,3 @@ func (o *OpChangeType) Validate(ctx context.Context, s *schema.Schema) error {
 	}
 	return nil
 }
-
-func duplicateColumnForTypeChange(ctx context.Context, conn *sql.DB, table *schema.Table, column schema.Column, newType string) error {
-	column.Name = TemporaryName(column.Name)
-	column.Type = newType
-
-	_, err := conn.ExecContext(ctx, fmt.Sprintf("ALTER TABLE %s ADD COLUMN %s",
-		pq.QuoteIdentifier(table.Name),
-		schemaColumnToSQL(column),
-	))
-
-	return err
-}