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RSpec matcher to control SQL queries made by block of code

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Test-Driven way of fighting N + 1 queries

RSpec matcher to control number of SQL queries executed by a block of code.

It wraps the answer at Stack Overflow by Ryan Bigg, which based on Active Support Notification and Instrumentation mechanisms.

For motivation and details see my blog post "Fighting the Hydra of N+1 queries" in the Martian Chronicles.

Gem Version Build Status Dependency Status Code Climate

Sponsored by Evil Martians

Installation

# Gemfile
gem "rspec-sqlimit"

Usage

The gem defines matcher exceed_query_limit that takes maximum number of SQL requests to be made inside the block.

require "rspec-sqlimit"

RSpec.describe "N+1 safety" do
  it "doesn't send unnecessary requests to db" do
    expect { User.create }.not_to exceed_query_limit(1)
  end
end

The above specification fails with the following description:

Failure/Error: expect { User.create }.not_to exceed_query_limit(1)

 Expected to run maximum 1 queries
 The following 3 queries were invoked:
    1) begin transaction (0.045 ms)
    2) INSERT INTO "users" DEFAULT VALUES (0.19 ms)
    3) commit transaction (148.935 ms)

You can restrict the matcher using regex:

require "rspec-sqlimit"

RSpec.describe "N+1 safety" do
  it "doesn't send unnecessary requests to db" do
    expect { User.create }.not_to exceed_query_limit(1).with(/^INSERT/)
  end
end

This time test passes.

When a specification with a restriction fails, you'll see an error as follows:

require "rspec-sqlimit"

RSpec.describe "N+1 safety" do
  it "doesn't send unnecessary requests to db" do
    expect { User.create }.not_to exceed_query_limit(1).with(/^INSERT/)
  end
end
Failure/Error: expect { User.create }.not_to exceed_query_limit(0).with(/INSERT/)

  Expected to run maximum 0 queries that match (?-mix:INSERT)
  The following 1 queries were invoked among others (see mark ->):
     1) begin transaction (0.072 ms)
  -> 2) INSERT INTO "users" DEFAULT VALUES (0.368 ms)
     3) commit transaction (147.559 ms)

Further Development

For now the gem uses unbinded Active Record queries in error descriptions. For example, when your query contains arguments, the error message will look like

require "rspec-sqlimit"

RSpec.describe "N+1 safety" do
  it "doesn't send unnecessary requests to db" do
    expect { User.create(name: "Joe") }.not_to exceed_query_limit(1)
  end
end
Failure/Error: expect { User.create }.not_to exceed_query_limit(0).with(/INSERT/)

  Expected to run maximum 0 queries that match (?-mix:INSERT)
  The following 1 queries were invoked among others (see mark ->):
     1) begin transaction (0.072 ms)
  -> 2) INSERT INTO "users" ("name") VALUES (?) (0.368 ms)
     3) commit transaction (147.559 ms)

This is because Active Record instrumentation hook keeps a query and bindings separately (under :sql and :binds keys). So the challenge is to bind arguments to the query in the report to make a debugging a bit simpler.

License

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

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