Add extra validation for the set NOT NULL op (#69)

Ensure that the column is not already marked as `NOT NULL`.

pkg/migrations/errors.go

@@ -36,6 +36,15 @@ func (e ColumnDoesNotExistError) Error() string {
 	return fmt.Sprintf("column %q does not exist on table %q", e.Name, e.Table)
 }
 
+type ColumnIsNotNullableError struct {
+	Table string
+	Name  string
+}
+
+func (e ColumnIsNotNullableError) Error() string {
+	return fmt.Sprintf("column %q on table %q is NOT NULL", e.Name, e.Table)
+}
+
 type IndexAlreadyExistsError struct {
 	Name string
 }

pkg/migrations/op_set_notnull.go

@@ -171,10 +171,15 @@ func (o *OpSetNotNull) Validate(ctx context.Context, s *schema.Schema) error {
 		return TableDoesNotExistError{Name: o.Table}
 	}
 
-	if table.GetColumn(o.Column) == nil {
+	column := table.GetColumn(o.Column)
+	if column == nil {
 		return ColumnDoesNotExistError{Table: o.Table, Name: o.Column}
 	}
 
+	if !column.Nullable {
+		return ColumnIsNotNullableError{Table: o.Table, Name: o.Column}
+	}
+
 	if o.Up == nil {
 		return FieldRequiredError{Name: "up"}
 	}

pkg/migrations/op_set_notnull_test.go

@@ -239,5 +239,51 @@ func TestSetNotNullValidation(t *testing.T) {
 			},
 			wantStartErr: migrations.ColumnDoesNotExistError{Table: "reviews", Name: "doesntexist"},
 		},
+		{
+			name: "column is nullable",
+			migrations: []migrations.Migration{
+				{
+					Name: "01_add_table",
+					Operations: migrations.Operations{
+						&migrations.OpCreateTable{
+							Name: "reviews",
+							Columns: []migrations.Column{
+								{
+									Name:       "id",
+									Type:       "serial",
+									PrimaryKey: true,
+								},
+								{
+									Name:     "username",
+									Type:     "text",
+									Nullable: false,
+								},
+								{
+									Name:     "product",
+									Type:     "text",
+									Nullable: false,
+								},
+								{
+									Name:     "review",
+									Type:     "text",
+									Nullable: false,
+								},
+							},
+						},
+					},
+				},
+				{
+					Name: "02_set_not_null",
+					Operations: migrations.Operations{
+						&migrations.OpSetNotNull{
+							Table:  "reviews",
+							Column: "review",
+							Up:     ptr("product || ' is good'"),
+						},
+					},
+				},
+			},
+			wantStartErr: migrations.ColumnIsNotNullableError{Table: "reviews", Name: "review"},
+		},
 	})
 }