Allow adding indexes to columns created in the same migration (#454)

Allow the `create_index` operation to add an index to a column that was
created by an operation earlier in the same migration.

The following migration would previously have failed to start:

```json
{
  "name": "43_multiple_ops",
  "operations": [
    {
      "create_table": {
        "name": "players",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "name",
            "type": "varchar(255)",
            "check": {
              "name": "name_length_check",
              "constraint": "length(name) > 2"
            }
          }
        ]
      }
    },
    {
      "add_column": {
        "table": "players",
        "column": {
          "name": "rating",
          "type": "integer",
          "comment": "hello world",
          "check": {
            "name": "rating_check",
            "constraint": "rating > 0 AND rating < 100"
          },
          "nullable": false
        }
      }
    },
    {
      "create_index": {
        "name": "idx_player_rating",
        "table": "players",
        "columns": [
          "rating"
        ]
      }
    }
  ]
}
```

As of this PR the migration can be started.

The above migration does not validate yet, but it can be started
successfully with the `--skip-validation` flag to the `start` command.

Part of #239

pkg/migrations/op_create_index.go

@@ -31,7 +31,9 @@ func (o *OpCreateIndex) Start(ctx context.Context, conn db.DB, latestSchema stri
 		stmt += fmt.Sprintf(" USING %s", string(*o.Method))
 	}
 
-	stmt += fmt.Sprintf(" (%s)", strings.Join(quoteColumnNames(o.Columns), ", "))
+	stmt += fmt.Sprintf(" (%s)", strings.Join(
+		quoteColumnNames(table.PhysicalColumnNamesFor(o.Columns...)), ", "),
+	)
 
 	if o.StorageParameters != nil {
 		stmt += fmt.Sprintf(" WITH (%s)", *o.StorageParameters)

pkg/migrations/op_create_index_test.go

@@ -315,7 +315,7 @@ func TestCreateIndexOnMultipleColumns(t *testing.T) {
 	}})
 }
 
-func TestCreateIndexOnTableCreatedInSameMigration(t *testing.T) {
+func TestCreateIndexOnObjectsCreatedInSameMigration(t *testing.T) {
 	t.Parallel()
 
 	ExecuteTests(t, TestCases{
@@ -361,5 +361,61 @@ func TestCreateIndexOnTableCreatedInSameMigration(t *testing.T) {
 				IndexMustExist(t, db, schema, "users", "idx_users_name")
 			},
 		},
-	}, roll.WithSkipValidation(true)) // TODO: Remove once this migration passes validation
+		{
+			name: "create index on newly created column",
+			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)",
+									Nullable: ptr(false),
+								},
+							},
+						},
+					},
+				},
+				{
+					Name: "02_add_column_and_index",
+					Operations: migrations.Operations{
+						&migrations.OpAddColumn{
+							Table: "users",
+							Column: migrations.Column{
+								Name:     "age",
+								Type:     "integer",
+								Nullable: ptr(true),
+							},
+							Up: "18",
+						},
+						&migrations.OpCreateIndex{
+							Name:    "idx_users_age",
+							Table:   "users",
+							Columns: []string{"age"},
+						},
+					},
+				},
+			},
+			afterStart: func(t *testing.T, db *sql.DB, schema string) {
+				// The index has been created on the underlying table.
+				IndexMustExist(t, db, schema, "users", "idx_users_age")
+			},
+			afterRollback: func(t *testing.T, db *sql.DB, schema string) {
+				// The index has been dropped from the the underlying table.
+				IndexMustNotExist(t, db, schema, "users", "idx_users_age")
+			},
+			afterComplete: func(t *testing.T, db *sql.DB, schema string) {
+				// The index has been created on the underlying table.
+				IndexMustExist(t, db, schema, "users", "idx_users_age")
+			},
+		},
+	}, roll.WithSkipValidation(true)) // TODO: Remove once these migrations pass validation
 }

pkg/schema/schema.go

@@ -253,6 +253,16 @@ func (t *Table) RenameColumn(from, to string) {
 	delete(t.Columns, from)
 }
 
+// PhysicalColumnNames returns the physical column names for the given virtual
+// column names
+func (t *Table) PhysicalColumnNamesFor(columnNames ...string) []string {
+	physicalNames := make([]string, 0, len(columnNames))
+	for _, cn := range columnNames {
+		physicalNames = append(physicalNames, t.GetColumn(cn).Name)
+	}
+	return physicalNames
+}
+
 // Make the Schema struct implement the driver.Valuer interface. This method
 // simply returns the JSON-encoded representation of the struct.
 func (s Schema) Value() (driver.Value, error) {