Ensure correct model equality semantics

They need to behave with data-object semantics

ee/app/models/design_management/design_at_version.rb

@@ -34,6 +34,14 @@ module DesignManagement
       "#{design.id}.#{version.id}"
     end
 
+    def ==(other)
+      return false unless other.is_a?(self.class)
+
+      other.id == id
+    end
+
+    alias_method :eql?, :==
+
     def self.lazy_find(id)
       BatchLoader.for(id).batch do |ids, callback|
         find(ids).each do |record|

ee/spec/models/design_management/design_at_version_spec.rb

@@ -18,6 +18,35 @@ describe DesignManagement::DesignAtVersion do
     end
   end
 
+  describe '#==' do
+    it 'has structural semantics' do
+      design = create(:design, issue: issue)
+      version = create(:design_version, designs: [design], issue: issue)
+
+      this = create(:design_at_version, design: design, version: version)
+      other = create(:design_at_version, design: design, version: version)
+
+      expect(this).to eq(other)
+    end
+
+    it 'does not admit false positives' do
+      design = create(:design, issue: issue)
+      version_a = create(:design_version, designs: [design])
+      version_b = create(:design_version, designs: [design])
+
+      this = create(:design_at_version, design: design, version: version_a)
+      other = create(:design_at_version, design: design, version: version_b)
+
+      expect(this).not_to eq(other)
+    end
+
+    it 'rejects objects with the same id and the wrong class' do
+      dav = create(:design_at_version)
+
+      expect(dav).not_to eq(OpenStruct.new(id: dav.id))
+    end
+  end
+
   describe 'status methods' do
     let!(:design_a) { create(:design, issue: issue) }
     let!(:design_b) { create(:design, issue: issue) }