Added has_and_belongs_to_many reification

HABTM associations are saved in the version_assocations table
with the foreign_key_name set to the association name. They are
saved if either the associated model is being paper trailed, or
if the option join_tables: [:association_name] is passed to the
has_paper_trail declaration. If the option is passed but the
associated model is not paper trailed, only the join model will
be saved. This means reification of HABTM would reify the
associated objects but in their current state. If the associated
model is paper trailed, this option does nothing, and the version
of the model at the time is reified.

.rubocop_todo.yml

@@ -12,7 +12,7 @@ Metrics/CyclomaticComplexity:
 # Offense count: 2
 # Configuration parameters: CountComments.
 Metrics/ModuleLength:
-  Max: 281
+  Max: 299
 
 # Offense count: 6
 Metrics/PerceivedComplexity:

lib/paper_trail/has_paper_trail.rb

@@ -57,6 +57,41 @@ def has_paper_trail(options = {})
         setup_model_for_paper_trail(options)
 
         setup_callbacks_from_options options[:on]
+
+        setup_callbacks_for_habtm options[:join_tables]
+      end
+
+      def update_for_callback(name, callback, model, assoc)
+        paper_trail_habtm = model.instance_variable_get(:@paper_trail_habtm) || {}
+        paper_trail_habtm.reverse_merge!(name => { removed: [], added: [] })
+        case callback
+        when :before_add
+          paper_trail_habtm[name][:added] |= [assoc.id]
+          paper_trail_habtm[name][:removed] -= [assoc.id]
+        when :before_remove
+          paper_trail_habtm[name][:removed] |= [assoc.id]
+          paper_trail_habtm[name][:added] -= [assoc.id]
+        end
+        model.instance_variable_set(:@paper_trail_habtm, paper_trail_habtm)
+      end
+
+      def setup_callbacks_for_habtm(join_tables)
+        @paper_trail_save_join_tables = Array.wrap(join_tables)
+        # Adds callbacks to record changes to habtm associations such that on
+        # save the previous version of the association (if changed) can be
+        # interpreted
+        reflect_on_all_associations(:has_and_belongs_to_many).
+          reject { |a| paper_trail_options[:skip].include?(a.name.to_s) }.
+          each do |a|
+            added_callback = lambda do |*args|
+              update_for_callback(a.name, :before_add, args[-2], args.last)
+            end
+            removed_callback = lambda do |*args|
+              update_for_callback(a.name, :before_remove, args[-2], args.last)
+            end
+            send(:"before_add_for_#{a.name}").send(:<<, added_callback)
+            send(:"before_remove_for_#{a.name}").send(:<<, removed_callback)
+          end
       end
 
       def setup_model_for_paper_trail(options = {})
@@ -442,6 +477,11 @@ def record_destroy
       # Saves associations if the join table for `VersionAssociation` exists.
       def save_associations(version)
         return unless PaperTrail.config.track_associations?
+        save_associations_belongs_to(version)
+        save_associations_has_and_belongs_to_many(version)
+      end
+
+      def save_associations_belongs_to(version)
         self.class.reflect_on_all_associations(:belongs_to).each do |assoc|
           assoc_version_args = {
             version_id: version.id,
@@ -463,6 +503,27 @@ def save_associations(version)
         end
       end
 
+      def save_associations_has_and_belongs_to_many(version)
+        # Use the :added and :removed keys to extrapolate the HABTM associations
+        # to before any changes were made
+        self.class.reflect_on_all_associations(:has_and_belongs_to_many).each do |a|
+          next unless
+            self.class.instance_variable_get(:@paper_trail_save_join_tables).include?(a.name) ||
+                a.klass.paper_trail_enabled_for_model?
+          assoc_version_args = {
+            version_id: version.id,
+            foreign_key_name: a.name
+          }
+          assoc_ids =
+            send(a.name).to_a.map(&:id) +
+            (@paper_trail_habtm.try(:[], a.name).try(:[], :removed) || []) -
+            (@paper_trail_habtm.try(:[], a.name).try(:[], :added) || [])
+          assoc_ids.each do |id|
+            PaperTrail::VersionAssociation.create(assoc_version_args.merge(foreign_key_id: id))
+          end
+        end
+      end
+
       def reset_transaction_id
         PaperTrail.transaction_id = nil
       end

lib/paper_trail/reifier.rb

@@ -14,6 +14,7 @@ def reify(version, options)
           has_one: false,
           has_many: false,
           belongs_to: false,
+          has_and_belongs_to_many: false,
           unversioned_attributes: :nil
         )
 
@@ -104,7 +105,8 @@ def prepare_array_for_has_many(array, options, versions)
           elsif version.event == "create"
             options[:mark_for_destruction] ? record.tap(&:mark_for_destruction) : nil
           else
-            version.reify(options.merge(has_many: false, has_one: false, belongs_to: false))
+            version.reify(options.merge(has_many: false, has_one: false, belongs_to: false,
+                                        has_and_belongs_to_many: false))
           end
         end
 
@@ -113,7 +115,8 @@ def prepare_array_for_has_many(array, options, versions)
         # associations.
         array.concat(
           versions.values.map { |v|
-            v.reify(options.merge(has_many: false, has_one: false, belongs_to: false))
+            v.reify(options.merge(has_many: false, has_one: false, belongs_to: false,
+                                  has_and_belongs_to_many: false))
           }
         )
 
@@ -128,6 +131,10 @@ def reify_associations(model, options, version)
         reify_belongs_tos version.transaction_id, model, options if options[:belongs_to]
 
         reify_has_manys version.transaction_id, model, options if options[:has_many]
+
+        if options[:has_and_belongs_to_many]
+          reify_has_and_belongs_to_many version.transaction_id, model, options
+        end
       end
 
       # Restore the `model`'s has_one associations as they were when this
@@ -154,7 +161,8 @@ def reify_has_ones(transaction_id, model, options = {})
               end
             end
           else
-            child = version.reify(options.merge(has_many: false, has_one: false, belongs_to: false))
+            child = version.reify(options.merge(has_many: false, has_one: false, belongs_to: false,
+                                                has_and_belongs_to_many: false))
             model.appear_as_new_record do
               without_persisting(child) do
                 model.send "#{assoc.name}=", child
@@ -181,7 +189,8 @@ def reify_belongs_tos(transaction_id, model, options = {})
                          assoc.klass.where(assoc.klass.primary_key => collection_key).first
                        else
                          version.reify(options.merge(has_many: false, has_one: false,
-                                                     belongs_to: false))
+                                                     belongs_to: false,
+                                                     has_and_belongs_to_many: false))
                        end
 
           model.send("#{assoc.name}=".to_sym, collection)
@@ -276,6 +285,38 @@ def reify_has_many_through(transaction_id, associations, model, options = {})
         end
       end
 
+      def reify_has_and_belongs_to_many(transaction_id, model, options = {})
+        model.class.reflect_on_all_associations(:has_and_belongs_to_many).each do |a|
+          papertrail_enabled = a.klass.paper_trail_enabled_for_model?
+          next unless
+            model.class.instance_variable_get(:@paper_trail_save_join_tables).include?(a.name) ||
+                papertrail_enabled
+
+          version_ids = PaperTrail::VersionAssociation.
+            where("foreign_key_name = ?", a.name).
+            where("version_id = ?", transaction_id).
+            pluck(:foreign_key_id)
+
+          model.send(a.name).proxy_association.target =
+            version_ids.map do |id|
+              if papertrail_enabled
+                version = a.klass.paper_trail_version_class.
+                  where("item_type = ?", a.klass.name).
+                  where("item_id = ?", id).
+                  where("created_at >= ? OR transaction_id = ?",
+                    options[:version_at], transaction_id).
+                  order("id").limit(1).first
+                if version
+                  next version.reify(options.merge(has_many: false, has_one: false,
+                                                   belongs_to: false,
+                                                   has_and_belongs_to_many: false))
+                end
+              end
+              a.klass.where(a.klass.primary_key => id).first
+            end
+        end
+      end
+
       # Given a `version`, return the class to reify. This method supports
       # Single Table Inheritance (STI) with custom inheritance columns.
       #

test/dummy/app/models/bar_habtm.rb

@@ -0,0 +1,4 @@
+class BarHabtm < ActiveRecord::Base
+  has_and_belongs_to_many :foo_habtms
+  has_paper_trail
+end

test/dummy/app/models/foo_habtm.rb

@@ -0,0 +1,4 @@
+class FooHabtm < ActiveRecord::Base
+  has_and_belongs_to_many :bar_habtms
+  has_paper_trail
+end

test/dummy/db/migrate/20110208155312_set_up_test_tables.rb

@@ -249,9 +249,27 @@ def up
     create_table :citations, force: true do |t|
       t.integer :quotation_id
     end
+
+    create_table :foo_habtms, force: true do |t|
+      t.string :name
+    end
+
+    create_table :bar_habtms, force: true do |t|
+      t.string :name
+    end
+
+    create_table :bar_habtms_foo_habtms, force: true, id: false do |t|
+      t.integer :foo_habtm_id
+      t.integer :bar_habtm_id
+    end
+    add_index :bar_habtms_foo_habtms, [:foo_habtm_id]
+    add_index :bar_habtms_foo_habtms, [:bar_habtm_id]
   end
 
   def down
+    drop_table :bar_habtms_foo_habtms
+    drop_table :foo_habtms
+    drop_table :bar_habtms
     drop_table :citations
     drop_table :quotations
     drop_table :animals

test/unit/associations_test.rb

@@ -891,4 +891,102 @@ class AssociationsTest < ActiveSupport::TestCase
       end
     end
   end
+
+  context "has_and_belongs_to_many associations" do
+    context "foo and bar" do
+      setup do
+        @foo = FooHabtm.create(name: "foo")
+        Timecop.travel 1.second.since
+      end
+
+      context "where the association is created between model versions" do
+        setup do
+          @foo.update_attributes(name: "foo1", bar_habtms: [BarHabtm.create(name: "bar")])
+        end
+
+        context "when reified" do
+          setup { @reified = @foo.versions.last.reify(has_and_belongs_to_many: true) }
+
+          should "see the associated as it was at the time" do
+            assert_equal 0, @reified.bar_habtms.length
+          end
+
+          should "not persist changes to the live association" do
+            assert_not_equal @reified.bar_habtms, @foo.reload.bar_habtms
+          end
+        end
+      end
+
+      context "where the association is changed between model versions" do
+        setup do
+          @foo.update_attributes(name: "foo2", bar_habtms: [BarHabtm.create(name: "bar2")])
+          Timecop.travel 1.second.since
+          @foo.update_attributes(name: "foo3", bar_habtms: [BarHabtm.create(name: "bar3")])
+        end
+
+        context "when reified" do
+          setup { @reified = @foo.versions.last.reify(has_and_belongs_to_many: true) }
+
+          should "see the association as it was at the time" do
+            assert_equal "bar2", @reified.bar_habtms.first.name
+          end
+
+          should "not persist changes to the live association" do
+            assert_not_equal @reified.bar_habtms.first, @foo.reload.bar_habtms.first
+          end
+        end
+
+        context "when reified with has_and_belongs_to_many: false" do
+          setup { @reified = @foo.versions.last.reify }
+
+          should "see the association as it is now" do
+            assert_equal "bar3", @reified.bar_habtms.first.name
+          end
+        end
+      end
+
+      context "where the association is destroyed between model versions" do
+        setup do
+          @foo.update_attributes(name: "foo2", bar_habtms: [BarHabtm.create(name: "bar2")])
+          Timecop.travel 1.second.since
+          @foo.update_attributes(name: "foo3", bar_habtms: [])
+        end
+
+        context "when reified" do
+          setup { @reified = @foo.versions.last.reify(has_and_belongs_to_many: true) }
+
+          should "see the association as it was at the time" do
+            assert_equal "bar2", @reified.bar_habtms.first.name
+          end
+
+          should "not persist changes to the live association" do
+            assert_not_equal @reified.bar_habtms.first, @foo.reload.bar_habtms.first
+          end
+        end
+      end
+
+      context "where the unassociated model changes" do
+        setup do
+          @bar = BarHabtm.create(name: "bar2")
+          @foo.update_attributes(name: "foo2", bar_habtms: [@bar])
+          Timecop.travel 1.second.since
+          @foo.update_attributes(name: "foo3", bar_habtms: [BarHabtm.create(name: "bar4")])
+          Timecop.travel 1.second.since
+          @bar.update_attributes(name: "bar3")
+        end
+
+        context "when reified" do
+          setup { @reified = @foo.versions.last.reify(has_and_belongs_to_many: true) }
+
+          should "see the association as it was at the time" do
+            assert_equal "bar2", @reified.bar_habtms.first.name
+          end
+
+          should "not persist changes to the live association" do
+            assert_not_equal @reified.bar_habtms.first, @foo.reload.bar_habtms.first
+          end
+        end
+      end
+    end
+  end
 end