gorm

PGAdapter and gorm

PGAdapter supports gorm with the pgx driver. This document shows how to use this sample application, and lists the limitations when working with gorm with PGAdapter.

The sample.go file contains a sample application using gorm with PGAdapter. Use this as a reference for features of gorm that are supported with PGAdapter. This sample assumes that the reader is familiar with gorm, and it is not intended as a tutorial for how to use gorm in general.

The sample is by default executed using the Cloud Spanner emulator. You can run the sample on the emulator with this command:

go run sample.go

You can also run the sample application on a real Cloud Spanner PostgreSQL database with this command:

go run sample.go -project my-project -instance my-instance -database my-database

Replace the project, instance, and database with your Cloud Spanner PostgreSQL database. The sample will automatically create the required tables for this sample.

Support Level

gorm can be used with Cloud Spanner PostgreSQL databases, but with limitations. Applications that have been developed with gorm for PostgreSQL will probably require modifications before they can be used with Cloud Spanner PostgreSQL databases. It is possible to develop new applications using gorm with Cloud Spanner PostgreSQL databases. These applications will also work with PostgreSQL without modifications.

See Limitations for a full list of limitations when working with gorm.

PGAdapter Docker

PGAdapter is started in a Docker test container by the sample application. Docker is therefore required to be installed on your system to run this sample.

Open Source PostgreSQL

This sample can also be executed on open-source PostgreSQL. PostgreSQL is started in a Docker test container by the sample application. Use this command to run the sample application on open-source PostgreSQL:

go run sample.go -postgres=true

Data Types

Cloud Spanner supports the following data types in combination with gorm.

PostgreSQL Type	gorm / go type
boolean	bool, sql.NullBool
bigint / int8	int64, sql.NullInt64
varchar	string, sql.NullString
text	string, sql.NullString
float8 / double precision	float64, sql.NullFloat64
numeric	decimal.NullDecimal
timestamptz / timestamp with time zone	time.Time, sql.NullTime
date	datatypes.Date
bytea	[]byte
jsonb	string
bool[]	pq.BoolArray, pgtype.BoolArray
bigint[]	pq.Int64Array, pgtype.Int8Array
varchar[] / text[]	pq.StringArray, pgtype.TextArray
float8[] / double precision[]	pq.Float64Array, pgtype.Float8Array
numeric[]	pgtype.NumericArray
timestamptz[] / timestamp with time zone[]	pgtype.TimestamptzArray
date[]	pgtype.DateArray
bytea[]	pgtype.ByteaArray
jsonb[]	pgtype.JSONBArray

Limitations

The following limitations are currently known:

Limitation	Workaround
Migrations	Cloud Spanner does not support the full PostgreSQL DDL dialect. Automated migrations using gorm are therefore not supported.
OnConflict	OnConflict clauses are not supported
Locking	Lock clauses (e.g. clause.Locking{Strength: "UPDATE"}) are not supported. These are generally speaking also not required, as the default isolation level that is used by Cloud Spanner is serializable.
Auto-save associations	Auto saved associations are not supported, as these will automatically use an OnConflict clause

Migrations

Migrations are not supported as Cloud Spanner does not support the full PostgreSQL DDL dialect. It is recommended to create the schema manually. See create_data_model.sql for the data model for this example.

Generated Primary Keys

Cloud Spanner supports bit-reversed sequences. These work as regular sequences, except that the value is bit-reversed before being returned to the user. That makes these values safe for use as a primary key in Cloud Spanner, and these will not cause hot-spotting. You can use the standard gorm.Model in combination with bit-reversed sequences.

Example model definition:

type TicketSale struct {
	gorm.Model
	Concert      Concert
	ConcertId    string
	CustomerName string
	Price        decimal.Decimal
	Seats        pq.StringArray `gorm:"type:text[]"`
}

Corresponding table and sequence definition:

create sequence if not exists ticket_sale_seq
    bit_reversed_positive
    skip range 1 1000
    start counter with 50000
;

create table if not exists ticket_sales (
    id bigint not null primary key default nextval('ticket_sale_seq'),
    concert_id       varchar not null,
    customer_name    varchar not null,
    price            decimal not null,
    seats            text[],
    created_at       timestamptz,
    updated_at       timestamptz,
    deleted_at       timestamptz,
    constraint fk_ticket_sales_concerts foreign key (concert_id) references concerts (id)
);

See also https://cloud.google.com/spanner/docs/reference/postgresql/data-definition-language#create_sequence

Generated Columns

Generated columns can be used, but Cloud Spanner does not support the RETURNING keyword. This means that gorm is not able to get the value of the generated column directly after it has been updated.

// FullName is generated by the database. The '->' marks this a read-only field. Preferably this field should also
// include a `default:(-)` annotation, as that would make gorm read the value back using a RETURNING clause. That is
// however currently not supported.
FullName string `gorm:"->;type:GENERATED ALWAYS AS (coalesce(concat(first_name,' '::varchar,last_name))) STORED;"`

OnConflict Clauses

OnConflict clauses are not supported by Cloud Spanner and should not be used. The following will therefore not work.

user := User{
    ID:   1,
    Name: "User Name",
}
// OnConflict is not supported and this will return an error.
db.Clauses(clause.OnConflict{DoNothing: true}).Create(&user)

Auto-save Associations

Auto-saving associations will automatically use an OnConflict clause in gorm. These are not supported. Instead, the parent entity of the association must be created before the child entity is created.

blog := Blog{
    ID:     1,
    Name:   "",
    UserID: 1,
    User: User{
        ID:   1,
        Name: "User Name",
    },
}
// This will fail, as the insert statement for User will use an OnConflict clause.
db.Create(&blog).Error

Instead, do the following:

user := User{
    ID:   1,
    Name: "User Name",
    Age:  20,
}
blog := Blog{
    ID:     1,
    Name:   "",
    UserID: 1,
}
db.Create(&user)
db.Create(&blog)

Locking

Locking clauses, like clause.Locking{Strength: "UPDATE"}, are not supported. These are generally speaking also not required, as Cloud Spanner uses isolation level serializable for read/write transactions.