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N + 1 Control

RSpec and Minitest matchers to prevent the N+1 queries problem.

Example output

Why yet another gem to assert DB queries?

Unlike other libraries (such as db-query-matchers, rspec-sqlimit, etc), with n_plus_one_control you don't have to specify exact expectations to control your code behaviour (e.g. expect { subject }.to query(2).times).

Such expectations are rather hard to maintain, 'cause there is a big chance of adding more queries, not related to the system under test.

NPlusOneControl works differently. It evaluates the code under consideration several times with different scale factors to make sure that the number of DB queries behaves as expected (i.e. O(1) instead of O(N)).

So, it's for performance testing and not feature testing.

Why not just use bullet?

Of course, it's possible to use Bullet in tests (see more here), but it's not a silver bullet: there can be both false positives and true negatives.

This gem was born after I've found myself not able to verify with a test yet another N+1 problem.

Sponsored by Evil Martians


Add this line to your application's Gemfile:

group :test do
  gem "n_plus_one_control"

And then execute:

$ bundle



First, add NPlusOneControl to your spec_helper.rb:

# spec_helper.rb
require "n_plus_one_control/rspec"


# Wrap example into a context with :n_plus_one tag
context "N+1", :n_plus_one do
  # Define `populate` callbacks which is responsible for data
  # generation (and whatever else).
  # It accepts one argument – the scale factor (read below)
  populate { |n| create_list(:post, n) }

  specify do
    expect { get :index }.to perform_constant_number_of_queries

NOTE: do not use memoized values within the expectation block!

# BAD – won't work!

subject { get :index }

specify do
  expect { subject }.to perform_constant_number_of_queries

Availables modifiers:

# You can specify the RegExp to filter queries.
# By default, it only considers SELECT queries.
expect { get :index }.to perform_constant_number_of_queries.matching(/INSERT/)

# You can also provide custom scale factors
expect { get :index }.to perform_constant_number_of_queries.with_scale_factors(10, 100)

Using scale factor in spec

Let's suppose your action accepts parameter, which can make impact on the number of returned records:

get :index, params: {per_page: 10}

Then it is enough to just change per_page parameter between executions and do not recreate records in DB. For this purpose, you can use current_scale method in your example:

context "N+1", :n_plus_one do
  before { create_list :post, 3 }

  specify do
    expect { get :index, params: {per_page: current_scale} }.to perform_constant_number_of_queries


First, add NPlusOneControl to your test_helper.rb:

# test_helper.rb
require "n_plus_one_control/minitest"

Then use assert_perform_constant_number_of_queries assertion method:

def test_no_n_plus_one_error
  populate = ->(n) { create_list(:post, n) }

  assert_perform_constant_number_of_queries(populate: populate) do
    get :index

You can also specify custom scale factors or filter patterns:

  populate: populate,
  scale_factors: [2, 5, 10]
) do
  get :index

  populate: populate,
  matching: /INSERT/
) do

It's possible to specify a filter via NPLUSONE_FILTER env var, e.g.:

NPLUSONE_FILTER = users bundle exec rake test

You can also specify populate as a test class instance method:

def populate(n)
  create_list(:post, n)

def test_no_n_plus_one_error
  assert_perform_constant_number_of_queries do
    get :index

As in RSpec, you can use current_scale factor instead of populate block:

def test_no_n_plus_one_error
  assert_perform_constant_number_of_queries do
    get :index, params: {per_page: current_scale}

With caching

If you use caching you can face the problem when first request performs more DB queries than others. The solution is:

# RSpec

context "N + 1", :n_plus_one do
  populate { |n| create_list :post, n }

  warmup { get :index } # cache something must be cached

  specify do
    expect { get :index }.to perform_constant_number_of_queries

# Minitest

def populate(n)
  create_list(:post, n)

def warmup
  get :index

def test_no_n_plus_one_error
  assert_perform_constant_number_of_queries do
    get :index

# or with params

def test_no_n_plus_one
  populate = ->(n) { create_list(:post, n) }
  warmup = -> { get :index }

  assert_perform_constant_number_of_queries population: populate, warmup: warmup do
    get :index

If your warmup and testing procs are identical, you can use:

expext { get :index }.to perform_constant_number_of_queries.with_warming_up # RSpec only


There are some global configuration parameters (and their corresponding defaults):

# Default scale factors to use.
# We use the smallest possible but representative scale factors by default.
NPlusOneControl.default_scale_factors = [2, 3]

# Print performed queries if true in the case of failure
# You can activate verbosity through env variable NPLUSONE_VERBOSE=1
NPlusOneControl.verbose = false

# Print table hits difference, for example:
#   Unmatched query numbers by tables:
#     users (SELECT): 2 != 3
#     events (INSERT): 1 != 2
self.show_table_stats = true

# Ignore matching queries

# ActiveSupport notifications event to track queries.
# We track ActiveRecord event by default,
# but can also track rom-rb events ('sql.rom') as well.
NPlusOneControl.event = "sql.active_record"

# configure transactional behavour for populate method
# in case of use multiple database connections
NPlusOneControl::Executor.tap do |executor|
  connections =

  executor.transaction_begin = -> do
    connections.each { |connection| connection.begin_transaction(joinable: false) }
  executor.transaction_rollback = -> do

# Provide a backtrace cleaner callable object used to filter SQL caller location to display in the verbose mode
# Set it to nil to disable tracing.
# In Rails apps, we use Rails.backtrace_cleaner by default.
NPlusOneControl.backtrace_cleaner = ->(locations_array) { do_some_filtering(locations_array) }

# You can also specify the number of backtrace lines to show.
# MOTE: It could be specified via NPLUSONE_BACKTRACE env var
NPlusOneControl.backtrace_length = 1

# Sometime queries could be too large to provide any meaningful insight.
# You can configure an output length limit for quries in verbose mode by setting the follwing option
# NOTE: It could be specified via NPLUSONE_TRUNCATE env var
NPlusOneControl.truncate_query_size = 100

How does it work?

Take a look at our Executor to figure out what's under the hood.

What's next?

  • More matchers.

It may be useful to provide more matchers/assertions, for example:

# Actually, that means that it is N+1))
assert_linear_number_of_queries { some_code }

# But we can tune it with `coef` and handle such cases as selecting in batches
assert_linear_number_of_queries(coef: 0.1) do
  Post.find_in_batches { some_code }

# probably, also make sense to add another curve types
assert_logarithmic_number_of_queries { some_code }
  • Support custom non-SQL events.

N+1 problem is not a database specific: we can have N+1 Redis calls, N+1 HTTP external requests, etc. We can make n_plus_one_control customizable to support these scenarios (technically, we need to make it possible to handle different payload in the event subscriber).

If you want to discuss or implement any of these, feel free to open an issue or propose a pull request.


# install deps
bundle install

# run tests
bundle exec rake


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.

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