Hackernoon logoDo Not Add Database Index If It Already Exists in Rails by@prathamesh

Do Not Add Database Index If It Already Exists in Rails

Author profile picture

@prathameshPrathamesh

Rails 6 added an option
if_not_exists
to the
create_table
method which will not create the table if it already exists.  This option is handy when we have added a table directly in production but now want to add it through a migration so that it is available in other environments as well.
Check out how the 
if_not_exists
 option works for creating tables here.
Let's look at a migration to understand this feature.
class CreateIssues < ActiveRecord::Migration[6.0]
  def change
    create_table :issues, if_not_exists: true do |t|
      t.integer   :comment_count
      t.string    :url, :repo_name, :user_name
      t.datetime  :last_touched_at
      t.integer   :number

      t.timestamps
    end
  end
end
This migration will not create the
issues
table if it exists already, as we have passed
if_not_exists: true
to the create_table method. Let's change the migration a little bit. We will now add a reference to
users
table in issues table because we want to create a reference to the
users 
table as well.
class AddIssuesAgainAgainAgain < ActiveRecord::Migration[6.0]
  def change
    create_table :issues, if_not_exists: true do |t|
      t.integer   :comment_count
      t.string    :url, :repo_name, :user_name
      t.datetime  :last_touched_at
      t.integer   :number

      t.references :user
      t.timestamps
    end
  end
end
If this migration is run twice, it results into following error.
➜  codetriage git:(master) ✗ be rake db:migrate
== 20200401140530 AddIssuesAgainAgainAgain: migrating =========================
-- create_table(:issues, {:if_not_exists=>true})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

Index name 'index_issues_on_user_id' on table 'issues' already exists
/Users/prathamesh/Projects/sources/codetriage/db/migrate/20200401140530_add_issues_again_again_again.rb:3:in `change'
/Users/prathamesh/.rbenv/versions/2.6.5/bin/bundle:23:in `load'
/Users/prathamesh/.rbenv/versions/2.6.5/bin/bundle:23:in `<main>'

Caused by:
ArgumentError: Index name 'index_issues_on_user_id' on table 'issues' already exists
/Users/prathamesh/Projects/sources/codetriage/db/migrate/20200401140530_add_issues_again_again_again.rb:3:in `change'
/Users/prathamesh/.rbenv/versions/2.6.5/bin/bundle:23:in `load'
/Users/prathamesh/.rbenv/versions/2.6.5/bin/bundle:23:in `<main>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)
We can see that it did not cause an error related to creating the table. The error was raised for existing index. But we do know that the index may exist. That's why we added the
if_not_exists: true
flag.
Then why is Rails trying to create the index when we clearly told it check if the index exists or not.
Well, turns out the
if_not_exists
flag was not getting passed to the query that Rails was generating for adding index. To understand this properly, let's take a look at queries generated by above migration.
CREATE TABLE IF NOT EXISTS `issues`
CREATE INDEX `index_issues_on_user_id` on `issues`
The
INDEX
query that was generated by the above migration was not having the
IF NOT EXISTS
clause so Rails was trying to create the index again even if the
if_not_exists: true
was passed.
Let's look at the code of
create_table
method in Rails to understand why this is happening. We will only see the relevant code related to adding indexes as the method is very big.
def create_table
  td = create_table_definition(table_name, **options.extract!(:temporary, :if_not_exists, :options, :as, :comment))
  ...
  
  result = execute schema_creation.accept td
  ...
  
  unless supports_indexes_in_create?
    td.indexes.each do |column_name, index_options|
      add_index(table_name, column_name, index_options)
    end
  end
  ...
  
  result    
end
This code if translated to plain English looks like this.
def create_table
  Generate SQL for CREATE TABLE
  result = Execute SQL for CREATE TABLE
  
  For each of the indexes -
   - Generate SQL for ADD INDEX
   - Execute SQL for ADD INDEX
  End
   
  return result
end
Looking at the error that we got related to existing index, it is pretty evident that
if_not_exists
option was not used while generating the SQL for index whereas it was used while generating the SQL for creating table.
Now the solution is straightforward. We need to make sure that Rails uses the
if_not_exists
option while generating the SQL for index.
The generation of SQL for indexes and its execution is handled by the
add_index
method. It looks like this.
def add_index(table_name, column_name, options = {})
  index_name, index_type, index_columns, index_options = add_index_options(table_name, column_name, **options)
  execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{index_columns})#{index_options}"
end
So if we pass the
if_not_exists
option from
create_table
to
add_index
which in turn passes it to
add_index_options
and which generates the proper SQL that we want, then our problem is solved!
That's what I did in this pull request https://github.com/rails/rails/pull/38555 and this issue is now fixed on Rails master.
How to handle this issue with Rails 6.0.2 and below
Support for
if_not_exists
was added in Rails 6.0.0. This change for supporting it for indexes is merged in Rails master but it is not yet released. If you are running into similar issue while using Rails 6.0.2 or below, you can simply skip the migration based on an environment check.
class AddIssuesAgainAgainAgain < ActiveRecord::Migration[6.0]
  def change
    return if Rails.env.production?
    
    create_table :issues, if_not_exists: true do |t|
      t.integer   :comment_count
      t.string    :url, :repo_name, :user_name
      t.datetime  :last_touched_at
      t.integer   :number

      t.references :user
      t.timestamps
    end
  end
end
In this way, if you are creating any indexes in the migration, you will not see errors about their presence if the migration is already run.
One last thing
While working on this fix, I realized that MySQL does not support
IF NOT EXISTS
clause for indexes. Bummer! To support this feature for MySQL adapter, we have to actually check if the index exists or not before trying to add it.
# activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb 

def add_index(table_name, column_name, options = {}) #:nodoc:
  return if options[:if_not_exists] && index_exists?(table_name, column_name, options)
  ..
  execute ..
end
One more thing!
The support for
if_not_exists
is also available to
add_index
method now. So you can just pass it as follows when trying to create standalone indexes.
add_index :issues, :user_id, if_not_exists: true
If you are interested in more such articles about Ruby on Rails, then subscribe to my newsletter.

Tags

The Noonification banner

Subscribe to get your daily round-up of top tech stories!