Class: RuboCop::Cop::Migration::ChangeColumn

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

Overview

Avoid changing column type that is in use.

Changing the type of a column causes the entire table to be rewritten. During this time, reads and writes are blocked in Postgres, and writes are blocked in MySQL and MariaDB.

Some changes don’t require a table rewrite and are safe in PostgreSQL:

Type Safe Changes
cidr Changing to inet
citext Changing to text if not indexed, changing to string with no :limit if not indexed
datetime Increasing or removing :precision, changing to timestamptz when session time zone is UTC in Postgres 12+
decimal Increasing :precision at same :scale, removing :precision and :scale
interval Increasing or removing :precision
numeric Increasing :precision at same :scale, removing :precision and :scale
string Increasing or removing :limit, changing to text, changing citext if not indexed
text Changing to string with no :limit, changing to citext if not indexed
time Increasing or removing :precision
timestamptz Increasing or removing :limit, changing to datetime when session time zone is UTC in Postgres 12+

And some in MySQL and MariaDB:

Type Safe Changes
string Increasing :limit from under 255 up to 255, increasing :limit from over 255 to the max

A safer approach is to:

  1. Create a new column
  2. Write to both columns
  3. Backfill data from the old column to the new column
  4. Move reads from the old column to the new column
  5. Stop writing to the old column
  6. Drop the old column

Examples:

# bad
class ChangeUsersSomeColumnType < ActiveRecord::Migration[7.0]
  def change
    change_column :users, :some_column, :new_type
  end
end

# good
class AddUsersAnotherColumn < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :another_column, :new_type
  end
end

class RemoveUsersSomeColumn < ActiveRecord::Migration[7.0]
  def change
    remove_column :users, :some_column
  end
end

Constant Summary collapse

MSG = 
'Avoid changing column type that is in use.'
RESTRICT_ON_SEND = 
i[
  change
  change_column
].freeze

Instance Method Summary collapse

Instance Method Details

#on_send(node) ⇒ void Also known as: on_csend

This method returns an undefined value.

Parameters:

  • node (RuboCop::AST::SendNode) 


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# File 'lib/rubocop/cop/migration/change_column.rb', line 74

def on_send(node)
  return unless bad?(node)

  add_offense(node)
end