Add with_buffer chain on rspec matcher

`with_buffer` takes an integer buffer
to allow for existing N+1 queries.

lib/n_plus_one_control/rspec/matcher.rb

@@ -16,6 +16,10 @@
     @warmup = true
   end
 
+  chain :with_buffer do |buffer|
+    @buffer = buffer
+  end
+
   match do |actual, *_args|
     raise ArgumentError, "Block is required" unless actual.is_a? Proc
 
@@ -37,9 +41,15 @@
 
     @queries = @matcher_execution_context.executor.call(&actual)
 
+    scale_factors = @queries.map(&:first)
     counts = @queries.map(&:last).map(&:size)
 
-    counts.max == counts.min
+    if scale_factors.max == scale_factors.min
+      counts.max == counts.min
+    else
+      buffer = @buffer || 0
+      (counts.max - counts.min) / (scale_factors.max - scale_factors.min) <= buffer
+    end
   end
 
   match_when_negated do |_actual|

spec/n_plus_one_control/rspec_spec.rb

@@ -23,6 +23,28 @@
     end
   end
 
+  context "when has N+1 with buffer", :n_plus_one do
+    populate { |n| create_list(:post, n) }
+
+    specify do
+      expect do
+        expect { Post.find_each { |p| p.user.name } }
+          .to perform_constant_number_of_queries.with_buffer(1)
+      end.not_to raise_error
+    end
+  end
+
+  context "when has N+1 exceeding buffer", :n_plus_one do
+    populate { |n| create_list(:post, n) }
+
+    specify do
+      expect do
+        expect { Post.find_each { |p| "#{p.user.name} #{p.category.name}" } }
+          .to perform_constant_number_of_queries.with_buffer(1)
+      end.to raise_error(RSpec::Expectations::ExpectationNotMetError)
+    end
+  end
+
   context "when context is missing" do
     specify do
       expect do
@@ -52,7 +74,7 @@
       expect do
         expect { Post.find_each { |p| p.user.name } }
           .to perform_constant_number_of_queries
-      end.to raise_error(RSpec::Expectations::ExpectationNotMetError, /select .+ from.*↳ .*rspec_spec.rb:53/im)
+      end.to raise_error(RSpec::Expectations::ExpectationNotMetError, /select .+ from.*↳ .*rspec_spec.rb:75/im)
     end
 
     context "when truncate size is specified" do
@@ -87,7 +109,7 @@
             .to perform_constant_number_of_queries
         end.to raise_error(
           RSpec::Expectations::ExpectationNotMetError,
-          /select .+ from.*↳.*rspec_spec.rb:86.*\n.*rspec_spec.rb:86/im
+          /select .+ from.*↳.*rspec_spec.rb:108.*\n.*rspec_spec.rb:108/im
         )
       end
     end
@@ -113,6 +135,15 @@
   context "with scale_factors", :n_plus_one do
     populate { |n| create_list(:post, n) }
 
+    specify do
+      expect { Post.preload(:user).find_each { |p| p.user.name } }
+        .to perform_constant_number_of_queries.with_scale_factors(1, 2)
+    end
+  end
+
+  context "with equal scale_factors", :n_plus_one do
+    populate { |n| create_list(:post, n) }
+
     specify do
       expect { Post.find_each { |p| p.user.name } }
         .to perform_constant_number_of_queries.with_scale_factors(1, 1)

spec/support/category.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+ActiveRecord::Schema.define do
+  create_table :categories do |t|
+    t.string :name
+  end
+end
+
+class Category < ActiveRecord::Base
+  has_many :post
+end
+
+FactoryGirl.define do
+  factory :category do
+    name "Fiction"
+  end
+end

spec/support/post.rb

@@ -4,16 +4,19 @@
   create_table :posts do |t|
     t.string :title
     t.integer :user_id
+    t.integer :category_id
   end
 end
 
 class Post < ActiveRecord::Base
   belongs_to :user
+  belongs_to :category
 end
 
 FactoryGirl.define do
   factory :post do
     title "Title"
     user
+    category
   end
 end