Class: RuboCop::Cop::DevDoc::Migration::AvoidColumnDefault

Inherits:
Base
  • Object
show all
Defined in:
lib/rubocop/cop/dev_doc/migration/avoid_column_default.rb

Overview

Avoid setting a ‘default:` value in migrations.

## Rationale Avoid adding business logic to the database. Keep it centralized in the application layer (controller or model) for easier maintenance and flexibility. A ‘default:` in a migration embeds a business-logic assumption into the database schema, which is harder to change later than code.

Instead of relying on a database default, set the value explicitly in the application — and for existing rows, backfill via a reversible migration that goes through model validations:

❌
class AddProfileCompletionRateToUsers < ActiveRecord::Migration[6.1]
  def change
    add_column :users, :profile_completion_rate, :float, default: 0.0
  end
end

✔
class AddProfileCompletionRateToUsers < ActiveRecord::Migration[6.1]
  def change
    add_column :users, :profile_completion_rate, :float

    reversible do |dir|
      dir.up do
        # Make sure Rails picks up the new column.
        User.reset_column_information

        User.where(profile_completion_rate: nil).find_each do |user|
          user.profile_completion_rate = 0.0
          # This may fail if existing records are invalid (e.g. nil required fields).
          # In that case, fix those records first rather than bypassing validation.
          user.save!
        end
      end
    end
  end
end

## Forms covered The cop catches ‘default:` set at column-creation time AND `default:` set later via `change_column_default(…, to: <non-nil>)`. Both are the same anti-pattern — a permanent default living in the schema. `change_column_default(…, to: nil)` (removing a default) is not flagged; that is the cleanup form.

## Exception (auto-detected) For performance reasons (large tables with millions of records) or when using ‘null: false`, you may temporarily set a default and then immediately remove it in the same migration. The cop suppresses the offense when it detects a matching `change_column_default …, to: nil` for the same table and column anywhere in the same method body (`def change` / `def up`), including inside `reversible do |dir| dir.up`.

The exception applies symmetrically:

  • ‘add_column … default: X` paired with `change_column_default … to: nil` → no offense

  • ‘change_column_default … to: X` paired with `change_column_default … to: nil` → no offense

    ✔ (no offense — two-step pattern auto-detected) add_column :users, :profile_completion_rate, :float, default: 0.0 change_column_default :users, :profile_completion_rate, from: 0.0, to: nil

Examples:

# bad
add_column :users, :score, :integer, default: 0

# bad
t.string :status, default: 'active'

# bad — permanent default set via change_column_default
change_column_default :users, :score, from: nil, to: 0

# good
add_column :users, :score, :integer

# good (temporary default immediately removed — two-step pattern)
add_column :users, :score, :integer, default: 0
change_column_default :users, :score, from: 0, to: nil

# good (removing an existing default)
change_column_default :users, :score, from: 0, to: nil

Constant Summary collapse

MSG = 
'Avoid setting `default:` in migrations. Keep business logic defaults in the application layer.'.freeze
MSG_CHANGE = 
'Avoid setting a non-nil default via `change_column_default`. ' \
'Keep business logic defaults in the application layer.'.freeze
COLUMN_METHODS = 
%i[
  string integer float boolean datetime date text binary decimal
  json jsonb bigint primary_key references belongs_to
].freeze

Instance Method Summary collapse

Instance Method Details

#on_send(node) ⇒ Object 



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# File 'lib/rubocop/cop/dev_doc/migration/avoid_column_default.rb', line 111

def on_send(node)
  if node.method?(:change_column_default)
    check_change_column_default(node)
  elsif column_method?(node)
    check_column_method(node)
  end
end