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Planning a Migration

Supporting repository

The change described in this section can be found in PR #6 in the supporting repository.

Planning a migration​

In this section, we will discuss how to plan a new schema migration when we make a change to our project's Ent schema. Consider we want to add a new field to our User entity, adding a new optional field named title:

ent/schema/user.go
// Fields of the User.
func (User) Fields() []ent.Field {
    return []ent.Field{
        field.String("name"),
        field.String("email"). // <-- Our new field
            Unique(),
        field.String("title").
            Optional(),
    }
}

After adding the new field, we need to rerun code-gen for our project:

go generate ./...

Next, we need to create a new migration file for our change using the Atlas CLI:

atlas migrate diff add_user_title \
  --dir "file://ent/migrate/migrations" \
  --to "ent://ent/schema" \
  --dev-url "docker://mysql/8/ent"

Observe a new file named 20221115101649_add_user_title.sql was created under the ent/migrate/migrations/ directory. This file contains the SQL statements to create the newly added title field in the users table:

ent/migrate/migrations/20221115101649_add_user_title.sql
-- modify "users" table
ALTER TABLE `users` ADD COLUMN `title` varchar(255) NULL;

Great! We've successfully used the Atlas CLI to automatically generate a new migration file for our change.

To apply the migration, we can run the following command:

atlas migrate apply --dir file://ent/migrate/migrations --url mysql://root:pass@localhost:3306/db

Atlas reports:

Migrating to version 20221115101649 from 20221114165732 (1 migrations in total):

  -- migrating version 20221115101649
    -> ALTER TABLE `users` ADD COLUMN `title` varchar(255) NULL;
  -- ok (36.152277ms)

  -------------------------
  -- 44.1116ms
  -- 1 migrations
  -- 1 sql statements

In the next section, we will discuss how to plan custom schema migrations for our project.