Counting the number of queries performed

Question

I\'d like to test that a certain piece of code performs as few SQL queries as possible.

ActiveRecord::TestCase seems to have its own assert_querie

Answer 1

My vision of Ryan's script (cleaned up a bit and wrapped in a matcher), hope it is still actual for someone:

I put this to spec/support/query_counter.rb

module ActiveRecord
  class QueryCounter

    attr_reader :query_count

    def initialize
      @query_count = 0
    end

    def to_proc
      lambda(&method(:callback))
    end

    def callback(name, start, finish, message_id, values)
      @query_count += 1 unless %w(CACHE SCHEMA).include?(values[:name])
    end

  end
end

and this to spec/support/matchers/exceed_query_limit.rb

RSpec::Matchers.define :exceed_query_limit do |expected|

  match do |block|
    query_count(&block) > expected
  end

  failure_message_for_should_not do |actual|
    "Expected to run maximum #{expected} queries, got #{@counter.query_count}"
  end

  def query_count(&block)
    @counter = ActiveRecord::QueryCounter.new
    ActiveSupport::Notifications.subscribed(@counter.to_proc, 'sql.active_record', &block)
    @counter.query_count
  end

end

Usage:

expect { MyModel.do_the_queries }.to_not exceed_query_limit(2)

Answer 2

Based on Jaime's answer, the following supports an assertion for the number of queries so far in the current test case, and will log the statements in case of failure. I think it's useful pragmatically to combine a SQL check like this with a functional test as it reduces the setup effort.

class ActiveSupport::TestCase

   ActiveSupport::Notifications.subscribe('sql.active_record') do |name, started, finished, unique_id, payload|
     (@@queries||=[]) << payload unless payload[:name].in? %w(CACHE SCHEMA)
   end

   def assert_queries_count(expected_count, message=nil)
     assert_equal expected_count, @@queries.size,
       message||"Expected #{expected_count} queries, but #{@@queries.size} queries occurred.#{@@queries[0,20].join(' ')}"
   end

   # common setup in a super-class (or use Minitest::Spec etc to do it another way)
   def setup
     @@queries = []
   end

end

Usage:

def test_something
   post = Post.new('foo')
   assert_queries_count 1 # SQL performance check
   assert_equal "Under construction", post.body # standard functional check
end

Note the query assertion should happen immediately in case the other assertions themselves trigger extra queries.