Up not required when adding serial columns (#432)

`serial` columns have an implicit default value so the `up` field is not
required.

Closes #193

docs/README.md

@@ -732,6 +732,8 @@ An add column operation creates a new column on an existing table.
 
 Default values are subject to the usual rules for quoting SQL expressions. In particular, string literals should be surrounded with single quotes.
 
+**NOTE:** As a special case, the `up` field can be omitted when adding `smallserial`, `serial` and `bigserial` columns. 
+
 Example **add column** migrations:
 
 * [03_add_column.json](../examples/03_add_column.json)

pkg/migrations/column.go

@@ -25,3 +25,13 @@ func (c *Column) IsPrimaryKey() bool {
 	}
 	return false
 }
+
+// HasImplicitDefault returns true if the column has an implicit default value
+func (c *Column) HasImplicitDefault() bool {
+	switch c.Type {
+	case "smallserial", "serial", "bigserial":
+		return true
+	default:
+		return false
+	}
+}

pkg/migrations/op_add_column.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"github.com/lib/pq"
+
 	"github.com/xataio/pgroll/pkg/db"
 	"github.com/xataio/pgroll/pkg/schema"
 )
@@ -171,7 +172,7 @@ func (o *OpAddColumn) Validate(ctx context.Context, s *schema.Schema) error {
 		}
 	}
 
-	if !o.Column.IsNullable() && o.Column.Default == nil && o.Up == "" {
+	if !o.Column.IsNullable() && o.Column.Default == nil && o.Up == "" && !o.Column.HasImplicitDefault() {
 		return FieldRequiredError{Name: "up"}
 	}
 

pkg/migrations/op_add_column_test.go

@@ -150,6 +150,109 @@ func TestAddColumn(t *testing.T) {
 			},
 			wantStartErr: migrations.BackfillNotPossibleError{Table: "users"},
 		},
+		{
+			name: "add serial columns",
+			migrations: []migrations.Migration{
+				{
+					Name: "01_add_table",
+					Operations: migrations.Operations{
+						&migrations.OpCreateTable{
+							Name: "users",
+							Columns: []migrations.Column{
+								{
+									Name: "id",
+									Type: "serial",
+									Pk:   ptr(true),
+								},
+								{
+									Name:   "name",
+									Type:   "varchar(255)",
+									Unique: ptr(true),
+								},
+							},
+						},
+					},
+				},
+				{
+					Name: "02_add_column",
+					Operations: migrations.Operations{
+						&migrations.OpAddColumn{
+							Table: "users",
+							Column: migrations.Column{
+								Name:     "counter_smallserial",
+								Type:     "smallserial",
+								Nullable: ptr(false),
+							},
+						},
+						&migrations.OpAddColumn{
+							Table: "users",
+							Column: migrations.Column{
+								Name:     "counter_serial",
+								Type:     "serial",
+								Nullable: ptr(false),
+							},
+						}, &migrations.OpAddColumn{
+							Table: "users",
+							Column: migrations.Column{
+								Name:     "counter_bigserial",
+								Type:     "bigserial",
+								Nullable: ptr(false),
+							},
+						},
+					},
+				},
+			},
+			afterStart: func(t *testing.T, db *sql.DB, schema string) {
+				// old and new views of the table should exist
+				ViewMustExist(t, db, schema, "01_add_table", "users")
+				ViewMustExist(t, db, schema, "02_add_column", "users")
+
+				// inserting via both the old and the new views works
+				MustInsert(t, db, schema, "01_add_table", "users", map[string]string{
+					"name": "Alice",
+				})
+				MustInsert(t, db, schema, "02_add_column", "users", map[string]string{
+					"name": "Bob",
+				})
+
+				// selecting from both the old and the new views works
+				resOld := MustSelect(t, db, schema, "01_add_table", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "name": "Alice"},
+					{"id": 2, "name": "Bob"},
+				}, resOld)
+				resNew := MustSelect(t, db, schema, "02_add_column", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "name": "Alice", "counter_smallserial": 1, "counter_serial": 1, "counter_bigserial": 1},
+					{"id": 2, "name": "Bob", "counter_smallserial": 2, "counter_serial": 2, "counter_bigserial": 2},
+				}, resNew)
+			},
+			afterRollback: func(t *testing.T, db *sql.DB, schema string) {
+				// The new column has been dropped from the underlying table
+				columnName := migrations.TemporaryName("age")
+				ColumnMustNotExist(t, db, schema, "users", columnName)
+
+				// The table's column count reflects the drop of the new column
+				TableMustHaveColumnCount(t, db, schema, "users", 2)
+			},
+			afterComplete: func(t *testing.T, db *sql.DB, schema string) {
+				// The new view still exists
+				ViewMustExist(t, db, schema, "02_add_column", "users")
+
+				// Inserting into the new view still works
+				MustInsert(t, db, schema, "02_add_column", "users", map[string]string{
+					"name": "Carl",
+				})
+
+				// Selecting from the new view still works
+				res := MustSelect(t, db, schema, "02_add_column", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "name": "Alice", "counter_smallserial": 1, "counter_serial": 1, "counter_bigserial": 1},
+					{"id": 2, "name": "Bob", "counter_smallserial": 2, "counter_serial": 2, "counter_bigserial": 2},
+					{"id": 3, "name": "Carl", "counter_smallserial": 3, "counter_serial": 3, "counter_bigserial": 3},
+				}, res)
+			},
+		},
 	})
 }