migrations

Ruby on Rails Migrations

Creating migration

• Use good name to migrations and follow the Ruby on Rails Guide.
• Use rails generate model to create table.
• Use migrations for update database values (access data). Good pattern is create a rake if the migration will be used more than one time.
• By default use add_index on relationships.

Coding Style and Conventions

• The following order is suggested when creating a migration: attributes come first grouped by data type; followed by associations and enumerations come last; all of them in alphabetical order. No need to group associations by integer or references, just order them alphabetically.
• No need to break lines except for the t.timestamps, this way the automatically generated migration may need no changes at all after creation.
• After the create_table block, add_foreign_key sentences come first and add_index sentences come last.
• Always add indexes for enums; it's easy to forget this if not done immediately after creating the migration. Add in the same line which the enum is declared, for example: t.integer :status, index: true.
• No need to break line after the the create_table block, this way the automatically generated migration may need no changes at all after creation.
• Break line between add_foreign_key and add_index sentences.
• It seems that the add_foreign_key was not automatically generated in some migrations due to older Rails version, please add it if necessary.
• Use t.references :model, index: true instead of t.integer :model_id, index: true whenever possible. For example: t.references :user, index: true (Rails 4+)
• For decimal data types, use precision: 10, scale: 2, for example: t.decimal :amount, precision: 10, scale: 2
• If the index name has more than 63 characters, remove the index: true and add it manually. The following pattern is suggested for naming the index: index_<table_name_plural_word>_on_<attribute_name>.
• For polymorphic associations, use t.references :<attribute_prefix>able, polymorphic: true, index: true whenever possible. Follow the Ruby on Rails Guide
  • If not possible due to index name length, remove the index: true sentence and manually add the index, for example: add_index :my_associations, [:associatable_id, :associatable_type], name: 'index_associations_on_associatable'

Cleanup migrations

When the db/migrations has too many files, is a good pattern consolidate all migrations in one file. All history will be in a file based on schema.rb and the command rake db:migrate works gracefuly. To do this, please use jalkoby/squasher.

Rollback

Please, be carefuly if command rake db:rollback is working correctly after create a migration. This saved my skin when it ran in production.

Brake migrations

To create a model and rename existing column type, create two migrations, one for each task. Please ensure that rake db:rollback is working for each one.

Frequently fix DB issues

• Use plentz/lol_dba to frequently fix index and other database issues.
• Use jenseng/immigrant to frequently add missing foreign keys of association tables.