Allow rename-only 'alter-column' operations on unbackfillable columns (…

…#341)

Allow rename-only 'alter column' operations on unbackfillable columns.
Rename operations don't require backfills so there is no reason to
impose such a restriction.

Fixes #340

pkg/migrations/op_alter_column.go

@@ -189,10 +189,12 @@ func (o *OpAlterColumn) Validate(ctx context.Context, s *schema.Schema) error {
 		return ColumnDoesNotExistError{Table: o.Table, Name: o.Column}
 	}
 
-	// Ensure that the column has a primary key defined on exactly one column.
-	err := checkBackfill(table)
-	if err != nil {
-		return err
+	// If the operation requires backfills (ie it isn't a rename-only operation),
+	// ensure that the column meets the requirements for backfilling.
+	if !o.isRenameOnly() {
+		if err := checkBackfill(table); err != nil {
+			return err
+		}
 	}
 
 	// If the column is being renamed, ensure that the target column name does

pkg/migrations/op_alter_column_test.go

@@ -507,7 +507,7 @@ func TestAlterColumnValidation(t *testing.T) {
 			wantStartErr: migrations.AlterColumnNoChangesError{Table: "posts", Column: "title"},
 		},
 		{
-			name: "table must have a primary key on exactly one column",
+			name: "if a backfill is required, the table must have a primary key on exactly one column",
 			migrations: []migrations.Migration{
 				{
 					Name: "01_add_table",
@@ -518,6 +518,31 @@ func TestAlterColumnValidation(t *testing.T) {
 						},
 					},
 				},
+				{
+					Name: "02_alter_column",
+					Operations: migrations.Operations{
+						&migrations.OpAlterColumn{
+							Table:    "orders",
+							Column:   "quantity",
+							Nullable: ptr(false),
+						},
+					},
+				},
+			},
+			wantStartErr: migrations.BackfillNotPossibleError{Table: "orders"},
+		},
+		{
+			name: "rename-only operations don't have primary key requirements",
+			migrations: []migrations.Migration{
+				{
+					Name: "01_add_table",
+					Operations: migrations.Operations{
+						&migrations.OpRawSQL{
+							Up:   "CREATE TABLE orders(id integer, order_id integer, quantity integer)",
+							Down: "DROP TABLE orders",
+						},
+					},
+				},
 				{
 					Name: "02_alter_column",
 					Operations: migrations.Operations{
@@ -529,7 +554,7 @@ func TestAlterColumnValidation(t *testing.T) {
 					},
 				},
 			},
-			wantStartErr: migrations.BackfillNotPossibleError{Table: "orders"},
+			wantStartErr: nil,
 		},
 	})
 }