Use relation.and instead of relation.merge whenever possible.

lib/cancan.rb

@@ -21,4 +21,5 @@
  require 'cancan/model_adapters/active_record_adapter'
  require 'cancan/model_adapters/active_record_4_adapter'
  require 'cancan/model_adapters/active_record_5_adapter'
+ require 'cancan/model_adapters/active_record_61_adapter'
 end

lib/cancan/model_adapters/active_record_5_adapter.rb

@@ -6,7 +6,7 @@ class ActiveRecord5Adapter < ActiveRecord4Adapter
  AbstractAdapter.inherited(self)
 
  def self.for_class?(model_class)
- version_greater_or_equal?('5.0.0') && model_class <= ActiveRecord::Base
+ version_greater_or_equal?('5.0.0') && version_lower?('6.1.0') && model_class <= ActiveRecord::Base
  end
 
  # rails 5 is capable of using strings in enum

lib/cancan/model_adapters/active_record_61_adapter.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module CanCan
+ module ModelAdapters
+ class ActiveRecord61Adapter < ActiveRecord5Adapter
+ AbstractAdapter.inherited(self)
+
+ def self.for_class?(model_class)
+ version_greater_or_equal?('6.1.0') && model_class <= ActiveRecord::Base
+ end
+
+ # rails 6.1 introduced `relation.and`
+ # which is more suitable for intersecting two relations then `relation.merge`
+ def database_records
+ return super unless override_scope
+
+ if @model_class.all.send(:structurally_incompatible_values_for, override_scope).empty?
+ @model_class.and(override_scope)
+ else
+ # wrap both side in subqeruy to satisfy structural compatibility requirements
+ wrap_model_subquery(@model_class.all).and(wrap_model_subquery(override_scope))
+ end
+ end
+
+ private
+
+ def wrap_model_subquery(scope)
+ real_model_class = @model_class.all.klass
+
+ real_model_class.where(real_model_class.primary_key => scope)
+ end
+ end
+ end
+end

spec/cancan/model_adapters/active_record_adapter_spec.rb

@@ -134,7 +134,12 @@ class User < ActiveRecord::Base
  end
 
  it 'is for only active record classes' do
- if ActiveRecord.version > Gem::Version.new('5')
+ if ActiveRecord.version > Gem::Version.new('6.1')
+ expect(CanCan::ModelAdapters::ActiveRecord61Adapter).to_not be_for_class(Object)
+ expect(CanCan::ModelAdapters::ActiveRecord61Adapter).to be_for_class(Article)
+ expect(CanCan::ModelAdapters::AbstractAdapter.adapter_class(Article))
+ .to eq(CanCan::ModelAdapters::ActiveRecord61Adapter)
+ elsif ActiveRecord.version > Gem::Version.new('5')
  expect(CanCan::ModelAdapters::ActiveRecord5Adapter).to_not be_for_class(Object)
  expect(CanCan::ModelAdapters::ActiveRecord5Adapter).to be_for_class(Article)
  expect(CanCan::ModelAdapters::AbstractAdapter.adapter_class(Article))