exceed_query_limit.rb

# frozen_string_literal: true

RSpec::Matchers.define :exceed_query_limit do |expected, pattern = nil|
  supports_block_expectations

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

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

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

query_counter.rb

# frozen_string_literal: true

# spec/support/active_record_query_counter.rb
module ActiveRecord
  class QueryCounter
    IGNORED_SQL = [
      /^PRAGMA (?!(table_info))/,
      /^SELECT currval/,
      /^SELECT CAST/,
      /^SELECT @@IDENTITY/,
      /^SELECT @@ROWCOUNT/,
      /^SAVEPOINT/,
      /^ROLLBACK TO SAVEPOINT/,
      /^RELEASE SAVEPOINT/,
      /^SHOW max_identifier_length/,
    ].freeze

    attr_reader :pattern, :queries_count

    def initialize(pattern)
      @pattern = pattern
      @queries_count = 0
    end

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

    def callback(_name, _start, _finish, _message_id, values)
      @queries_count += 1 if !ignore?(values) && match_pattern?(values)
    end

    def ignore?(values)
      %w[CACHE SCHEMA].include?(values[:name]) ||
        IGNORED_SQL.any? { |query| values[:sql] =~ query }
    end

    def match_pattern?(values)
      return true if @pattern.nil?

      values[:sql] =~ @pattern 
    end
  end
end

some_spec.rb

it "checks for N+1 on querying" do
  # .with_some_associations must perform includes with JOIN and perform only one SELECT query
  expect { SomeModel.with_some_associations.first }.not_to exceed_query_limit(1)
end

it "checks count of UPDATE statements" do
  # just an example
  expect { MySyperRepository.create_invocve!}.not_to exceed_query_limit(2, /^UPDATE/)
end