64 lines
2.1 KiB
Markdown
64 lines
2.1 KiB
Markdown
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---
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stage: Data Stores
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group: Database
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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---
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# Single Table Inheritance
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**Summary:** don't use Single Table Inheritance (STI), use separate tables
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instead.
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Rails makes it possible to have multiple models stored in the same table and map
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these rows to the correct models using a `type` column. This can be used to for
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example store two different types of SSH keys in the same table.
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While tempting to use one should avoid this at all costs for the same reasons as
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outlined in the document ["Polymorphic Associations"](polymorphic_associations.md).
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## Solution
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The solution is very simple: just use a separate table for every type you'd
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otherwise store in the same table. For example, instead of having a `keys` table
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with `type` set to either `Key` or `DeployKey` you'd have two separate tables:
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`keys` and `deploy_keys`.
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## In migrations
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Whenever a model is used in a migration, single table inheritance should be disabled.
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Due to the way Rails loads associations (even in migrations), failing to disable STI
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could result in loading unexpected code or associations which may cause unintended
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side effects or failures during upgrades.
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```ruby
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class SomeMigration < Gitlab::Database::Migration[2.0]
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class Services < MigrationRecord
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self.table_name = 'services'
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self.inheritance_column = :_type_disabled
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end
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def up
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...
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```
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If nothing needs to be added to the model other than disabling STI or `EachBatch`,
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use the helper `define_batchable_model` instead of defining the class.
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This ensures that the migration loads the columns for the migration in isolation,
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and the helper disables STI by default.
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```ruby
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class EnqueueSomeBackgroundMigration < Gitlab::Database::Migration[1.0]
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disable_ddl_transaction!
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def up
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define_batchable_model('services').select(:id).in_batches do |relation|
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jobs = relation.pluck(:id).map do |id|
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['ExtractServicesUrl', [id]]
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end
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BackgroundMigrationWorker.bulk_perform_async(jobs)
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end
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end
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...
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```
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