feat: add and drop IDENTITY

FYI, identity is independent of primary keys and vice versa.
It is only a method of generating new values for a column.
Can be created and dropped in any order, as long as
the column to add it to is declared NOT NULL

dialect/pgdialect/alter_table.go

@@ -185,43 +185,50 @@ func (m *migrator) changeColumnType(fmter schema.Formatter, b []byte, colDef *mi
 		return b, err
 	}
 
+	// alterColumn never re-assigns err, so there is no need to check for err != nil after calling it
 	var i int
 	appendAlterColumn := func() {
 		if i > 0 {
-			b = append(b, ", "...)
+			b = append(b, ","...)
 		}
 		b = append(b, " ALTER COLUMN "...)
-		b, err = bun.Ident(colDef.Column).AppendQuery(fmter, b)
+		b, _ = bun.Ident(colDef.Column).AppendQuery(fmter, b)
 		i++
 	}
 
 	got, want := colDef.From, colDef.To
 
 	if want.SQLType != got.SQLType {
-		if appendAlterColumn(); err != nil {
-			return b, err
-		}
+		appendAlterColumn()
 		b = append(b, " SET DATA TYPE "...)
 		if b, err = want.AppendQuery(fmter, b); err != nil {
 			return b, err
 		}
 	}
 
+	// Column must be declared NOT NULL before identity can be added.
+	// Although PG can resolve the order of operations itself, we make this explicit in the query.
 	if want.IsNullable != got.IsNullable {
-		if appendAlterColumn(); err != nil {
-			return b, err
-		}
+		appendAlterColumn()
 		if !want.IsNullable {
 			b = append(b, " SET NOT NULL"...)
 		} else {
 			b = append(b, " DROP NOT NULL"...)
 		}
 	}
 
-	if want.DefaultValue != got.DefaultValue {
-		if appendAlterColumn(); err != nil {
-			return b, err
+	if want.IsIdentity != got.IsIdentity {
+		appendAlterColumn()
+		if !want.IsIdentity {
+			b = append(b, " DROP IDENTITY"...)
+		} else {
+			b = append(b, " ADD"...)
+			b = appendGeneratedAsIdentity(b)
 		}
+	}
+
+	if want.DefaultValue != got.DefaultValue {
+		appendAlterColumn()
 		if want.DefaultValue == "" {
 			b = append(b, " DROP DEFAULT"...)
 		} else {

dialect/pgdialect/dialect.go

@@ -123,5 +123,10 @@ func (d *Dialect) AppendUint64(b []byte, n uint64) []byte {
 }
 
 func (d *Dialect) AppendSequence(b []byte, _ *schema.Table, _ *schema.Field) []byte {
+	return appendGeneratedAsIdentity(b)
+}
+
+// appendGeneratedAsIdentity appends GENERATED BY DEFAULT AS IDENTITY to the column definition.
+func appendGeneratedAsIdentity(b []byte) []byte {
 	return append(b, " GENERATED BY DEFAULT AS IDENTITY"...)
 }

internal/dbtest/migrate_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 	"github.com/uptrace/bun"
+	"github.com/uptrace/bun/dialect/sqltype"
 	"github.com/uptrace/bun/migrate"
 	"github.com/uptrace/bun/migrate/sqlschema"
 )
@@ -206,6 +207,7 @@ func TestAutoMigrator_Run(t *testing.T) {
 		{testForceRenameFK},
 		{testRenameColumnRenamesFK},
 		{testChangeColumnType_AutoCast},
+		{testIdentity},
 	}
 
 	testEachDB(t, func(t *testing.T, dbName string, db *bun.DB) {
@@ -660,7 +662,50 @@ func testChangeColumnType_AutoCast(t *testing.T, db *bun.DB) {
 	// Assert
 	state := inspect(ctx)
 	cmpTables(t, db.Dialect().(sqlschema.InspectorDialect), wantTables, state.Tables)
-	// require.Equal(t, wantTables, state.Tables
+}
+
+func testIdentity(t *testing.T, db *bun.DB) {
+	type TableBefore struct {
+		bun.BaseModel `bun:"table:table"`
+		A             int64 `bun:",notnull,identity"`
+		B             int64
+	}
+
+	type TableAfter struct {
+		bun.BaseModel `bun:"table:table"`
+		A             int64 `bun:",notnull"`
+		B             int64 `bun:",notnull,identity"`
+	}
+
+	wantTables := []sqlschema.Table{
+		{
+			Schema: db.Dialect().DefaultSchema(),
+			Name:   "table",
+			Columns: map[string]sqlschema.Column{
+				"a": {
+					SQLType:    sqltype.BigInt,
+					IsIdentity: false, // <- drop IDENTITY
+				},
+				"b": {
+					SQLType:    sqltype.BigInt,
+					IsIdentity: true, // <- add IDENTITY
+				},
+			},
+		},
+	}
+
+	ctx := context.Background()
+	inspect := inspectDbOrSkip(t, db)
+	mustResetModel(t, ctx, db, (*TableBefore)(nil))
+	m := newAutoMigrator(t, db, migrate.WithModel((*TableAfter)(nil)))
+
+	// Act
+	err := m.Run(ctx)
+	require.NoError(t, err)
+
+	// Assert
+	state := inspect(ctx)
+	cmpTables(t, db.Dialect().(sqlschema.InspectorDialect), wantTables, state.Tables)
 }
 
 // // TODO: rewrite these tests into AutoMigrator tests, Diff should be moved to migrate/internal package

migrate/diff.go

@@ -356,9 +356,8 @@ func (d *detector) detectColumnChanges(current, target sqlschema.Table, checkTyp
 			continue
 		}
 
-		// Find the column with the same type and the
 		for cName, cCol := range current.Columns {
-			if !d.equalColumns(tCol, cCol) {
+			if _, keep := target.Columns[cName]; keep || !d.equalColumns(tCol, cCol) {
 				continue
 			}
 			d.changes.Add(&RenameColumn{

migrate/operations.go

@@ -206,6 +206,7 @@ func (op *DropConstraint) GetReverse() Operation {
 	}
 }
 
+// Change column type.
 type ChangeColumnType struct {
 	Schema string
 	Table  string