Migrations

Migrations are a feature that allows to manage database schema via Ruby. They come with some command line facilities that allow to perform database operations or to generate migrations.

Migrations are only available if our application uses the SQL adapter.

Anatomy Of A Migration

Migrations are Ruby files stored by default under db/migrations. Their name is composed by a UTC timestamp and a snake case name (eg db/migrations/20150621165604_create_books.rb).

Hanami::Model.migration do
  change do
    create_table :books do
      primary_key :id
      foreign_key :author_id, :authors, on_delete: :cascade, null: false

      column :code,  String,  null: false, unique: true, size: 128
      column :title, String,  null: false
      column :price, Integer, null: false, default: 100 # cents

      check { price > 0 }
    end
  end
end

We use a create_table block to define the schema of that table.

The first line is primary_key :id, which is a shortcut to create an autoincrement integer column.

There is a foreign key definition with cascade deletion. The first argument is the name of the local column (books.author_id), while the second is the referenced table.

Then we have three lines for columns. The first argument that we pass to column is the name, then the type. The type can be a Ruby type such as String or Integer or a string that represents the native database type (eg. "varchar(32)" or "text[]").

As a last optional argument there is a Hash that specifies some extra details for the column. For instance NULL or uniqueness constraints, the size (for strings) or the default value.

The final line defines a database check to ensure that price will always be greater than zero.

Up/Down

When we "migrate" a database we are going into an "up" direction because we're adding alterations to it. Migrations modifications can be rolled back ("down" direction).

When we use change in our migrations, we're implicitly describing "up" modifications. Their counterpart can be inferred by Hanami::Model when we migrate "down" our database.

Imagine we have the following code:

Hanami::Model.migration do
  change do
    create_table :books do
      # ...
    end
  end
end

When we use create_table, Hanami::Model will use drop_table in case we want to rollback this migration.

In case we want to have concrete code for our "down" policy, we can use up and down blocks.

Hanami::Model.migration do
  up do
    create_table :books do
      # ...
    end
  end

  down do
    drop_table :books
  end
end

To learn how to use migrations in command line, please have a look at this article.

References

Hanami::Model uses Sequel under the hood as database migration engine. If there is any aspect that isn't covered by our documentation or tests, please refer to Sequel documentation.


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