Identify database issues before they hit production.
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gregnavis Implement missing_unique_indexes
This commit implements a task detecting missing unique indexes.
Currently, it support multi-column and scoped uniqueness. It also fixes
an issue with Temping that leaked temporary model classes defining in
the test suite.
Latest commit 098880e Apr 21, 2018

Active Record Doctor

Active Record Doctor helps to keep the database in a good shape. Currently, it can:

  • index unindexed foreign keys
  • detect extraneous indexes
  • detect unindexed deleted_at columns
  • detect missing foreign key constraints
  • detect models referencing undefined tables
  • detect uniqueness validations not backed by an unique index

More features coming soon!

Want to suggest a feature? Just shoot me an email.

Build Status


The preferred installation method is adding active_record_doctor to your Gemfile:

gem 'active_record_doctor', group: :development

Then run:

bundle install


Indexing Unindexed Foreign Keys

Foreign keys should be indexed unless it's proven ineffective. However, Rails makes it easy to create an unindexed foreign key. Active Record Doctor can automatically generate database migrations that add the missing indexes. It's a three-step process:

  1. Generate a list of unindexed foreign keys by running
rake active_record_doctor:unindexed_foreign_keys > unindexed_foreign_keys.txt
  1. Remove columns that should not be indexed from unindexed_foreign_keys.txt as a column can look like a foreign key (i.e. end with _id) without being one.

  2. Generate the migrations

rails generate active_record_doctor:add_indexes unindexed_foreign_keys.txt
  1. Run the migrations
rake db:migrate

Removing Extraneous Indexes

Let me illustrate with an example. Consider a users table with columns first_name and last_name. If there are two indexes:

  • A two-column index on last_name, first_name.
  • A single-column index on last_name.

Then the latter index can be dropped as the former can play its role. In general, a multi-column index on column_1, column_2, ..., column_n can replace indexes on:

  • column_1
  • column_1, column_2
  • ...
  • column_1, column_2, ..., column_(n - 1)

To discover such indexes automatically just follow these steps:

  1. List extraneous indexes by running:
rake active_record_doctor:extraneous_indexes
  1. Confirm that each of the indexes can be indeed dropped.

  2. Create a migration to drop the indexes.

The indexes aren't dropped automatically because there's usually just a few of them and it's a good idea to double-check that you won't drop something necessary.

Also, extra indexes on primary keys are considered extraneous too and will be reported.

Note that a unique index can never be replaced by a non-unique one. For example, if there's a unique index on users.login and a non-unique index on users.login, users.domain then the tool will not suggest dropping users.login as it could violate the uniqueness assumption.

Detecting Unindexed deleted_at Columns

If you soft-delete some models (e.g. with paranoia) then you need to modify your indexes to include only non-deleted rows. Otherwise they will include logically non-existent rows. This will make them larger and slower to use. Most of the time they should only cover columns satisfying deleted_at IS NULL.

active_record_doctor can automatically detect indexes on tables with a deleted_at column. Just run:

rake active_record_doctor:unindexed_soft_delete

This will print a list of indexes that don't have the deleted_at IS NULL clause. Currently, active_record_doctor cannot automatically generate appropriate migrations. You need to do that manually.

Detecting Missing Foreign Key Constraints

If users.profile_id references a row in profiles then this can be expressed at the database level with a foreign key constraint. It forces users.profile_id to point to an existing row in profiles. The problem is that in many legacy Rails apps the constraint isn't enforced at the database level.

active_record_doctor can automatically detect foreign keys that could benefit from a foreign key constraint (a future version will generate a migrations that add the constraint; for now, it's your job). You can obtain the list of foreign keys with the following command:

rake active_record_doctor:missing_foreign_keys

The output will look like:

users profile_id
comments user_id article_id

Tables are listed one per line. Each line starts with a table name followed by column names that should have a foreign key constraint. In the example above, users.profile_id, comments.user_id, and comments.article_id lack a foreign key constraint.

In order to add a foreign key constraint to users.profile_id use the following migration:

class AddForeignKeyConstraintToUsersProfileId < ActiveRecord::Migration
  def change
    add_foreign_key :users, :profiles

Detecting Models Referencing Undefined Tables

Active Record guesses the table name based on the class name. There are a few cases where the name can be wrong (e.g. you forgot to commit a migration or changed the table name). Active Record Doctor can help you identify these cases before they hit production.

The only think you need to do is run:

rake active_record_doctor:undefined_table_references

If there a model references an undefined table then you'll see a message like this:

The following models reference undefined tables:
  Contract (the table contract_records is undefined)

On top of that rake will exit with status code of 1. This allows you to use this check as part of your Continuous Integration pipeline.

Detecting Uniqueness Validations not Backed by an Index

A model-level uniqueness validations should be backed by a database index in order to be robust. Otherwise you risk inserting duplicate values under heavy load.

In order to detect such validations run:

rake active_record_doctor:missing_unique_indexes

If there are such indexes then the command will print:

The following indexes should be created to back model-level uniqueness validations:
  users: email

This means that you should create a unique index on


This gem is developed and maintained by Greg Navis.