MONGOID-5344 Using concat on has_and_belongs_to_many ids default value is not working the first time (#5268)

* MONGOID-5344 Using concat on has_and_belongs_to_many ids default value is not working the first time

* MONGOID-5344 add has_many test

Author: neilshweky

lib/mongoid/document.rb

@@ -309,7 +309,7 @@ def instantiate(attrs = nil, selected_fields = nil, &block)
       #
       # @api private
       def instantiate_document(attrs = nil, selected_fields = nil, execute_callbacks: true)
-        attributes = attrs || {}
+        attributes = attrs&.to_h || {}
         doc = allocate
         doc.__selected_fields = selected_fields
         doc.instance_variable_set(:@attributes, attributes)

spec/mongoid/association/referenced/has_and_belongs_to_many/proxy_spec.rb

@@ -3796,4 +3796,31 @@ class Distributor
       expect(Breed.aliased_associations["dog_ids"]).to eq("dogs")
     end
   end
+
+  # This test is for MONGOID-5344 which tests that the initial call to
+  # signature_ids refers to the same array as subsequent calls to signature_ids.
+  # Prior to the change in that ticket, this test broke because the array
+  # returned from write_attribute (which is triggered the first time the
+  # foreign key array is referenced, to set the default), refers to a different
+  # array to the one stored in the attributes hash. This happened because,
+  # when retrieving a document from the database, the attributes hash is actually
+  # a BSON::Document, which applies a transformation to the array before
+  # storing it.
+  context "when executing concat on foreign key array from the db" do
+    before do
+      HabtmmContract.create!
+      HabtmmSignature.create!
+    end
+
+    let!(:contract) { HabtmmContract.first }
+    let!(:signature) { HabtmmSignature.first }
+
+    before do
+      contract.signature_ids.concat([signature.id])
+    end
+
+    it "works on the first attempt" do
+      expect(contract.signature_ids).to eq([signature.id])
+    end
+  end
 end

spec/mongoid/association/referenced/has_many/proxy_spec.rb

@@ -4122,4 +4122,22 @@
       expect(Artist.aliased_fields["band"]).to eq("band_id")
     end
   end
+
+  context "when executing concat on foreign key array from the db" do
+    before do
+      Agent.create!
+      Basic.create!
+    end
+
+    let!(:agent) { Agent.first }
+    let!(:basic) { Basic.first }
+
+    before do
+      agent.basic_ids.concat([basic.id])
+    end
+
+    it "works on the first attempt" do
+      expect(agent.basic_ids).to eq([basic.id])
+    end
+  end
 end

spec/mongoid/attributes_spec.rb

@@ -1676,6 +1676,49 @@
         end
       end
     end
+
+    context "when comparing the object_ids of the written value" do
+
+      before do
+        Person.create!
+      end
+
+      let(:person) do
+        Person.first
+      end
+
+      context "when the field is not resizable" do
+        let(:test) do
+          person.write_attribute(:test, "aliased field to test")
+        end
+
+        it "has the same object_id as the attributes hash value" do
+          expect(test.object_id).to eq(person.test.object_id)
+        end
+      end
+
+      context "when the field is resizable" do
+
+        let(:arrays) do
+          person.write_attribute(:arrays, [])
+        end
+
+        it "has the same object_id as the attributes hash value" do
+          expect(arrays.object_id).to eq(person.arrays.object_id)
+        end
+      end
+
+      context "when the field is a HABTM foreign key array" do
+
+        let(:preference_ids) do
+          person.write_attribute(:preference_ids, [])
+        end
+
+        it "has the same object_id as the attributes hash value" do
+          expect(preference_ids.object_id).to eq(person.preference_ids.object_id)
+        end
+      end
+    end
   end
 
   describe "#typed_value_for" do