Planning a Migration

import Tabs from '@theme/Tabs'; import TabItem from '@theme/TabItem';

:::info Supporting repository

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

:::

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:

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

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

shell

go generate ./...

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

shell

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

</TabItem> <TabItem value="maria">

shell

atlas migrate diff add_user_title \
  --dir "file://ent/migrate/migrations" \
  --to "ent://ent/schema" \
  --dev-url "docker://mariadb/latest/test"

</TabItem> <TabItem value="postgres">

shell

atlas migrate diff add_user_title \
  --dir "file://ent/migrate/migrations" \
  --to "ent://ent/schema" \
  --dev-url "docker://postgres/15/test?search_path=public"

</TabItem> <TabItem value="sqlite">

shell

atlas migrate diff add_user_title \
  --dir "file://ent/migrate/migrations" \
  --to "ent://ent/schema" \
  --dev-url "sqlite://file?mode=memory&_fk=1"

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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:

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:

shell

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

Atlas reports:

shell

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.