Fix issues mentioned but not closed for JIRA

The `ReferenceExtractor` would return an array of `ExternalIssue`
objects, and then perform `Array#-` to remove the issues
closed. `ExternalIssue`s had `==` defined, but not `hash` or `eql?`,
which are used by `Array#-`.

app/models/external_issue.rb

@@ -24,6 +24,11 @@ class ExternalIssue
   def ==(other)
     other.is_a?(self.class) && (to_s == other.to_s)
   end
+  alias_method :eql?, :==
+
+  def hash
+    [self.class, to_s].hash
+  end
 
   def project
     @project

changelogs/unreleased/fix-mentioned-issues-for-external-trackers.yml

@@ -0,0 +1,4 @@
+---
+title: Fix issues mentioned but not closed for external issue trackers
+merge_request:
+author:

spec/factories/services.rb

@@ -12,4 +12,13 @@ FactoryGirl.define do
       token: 'a' * 40,
     })
   end
+
+  factory :jira_service do
+    project factory: :empty_project
+    active true
+    properties(
+      url: 'https://jira.example.com',
+      project_key: 'jira-key'
+    )
+  end
 end

spec/models/external_issue_spec.rb

@@ -42,4 +42,12 @@ describe ExternalIssue, models: true do
       expect(issue.project_id).to eq(project.id)
     end
   end
+
+  describe '#hash' do
+    it 'returns the hash of its [class, to_s] pair' do
+      issue_2 = described_class.new(issue.to_s, project)
+
+      expect(issue.hash).to eq(issue_2.hash)
+    end
+  end
 end

spec/models/merge_request_spec.rb

@@ -346,6 +346,23 @@ describe MergeRequest, models: true do
 
       expect(subject.issues_mentioned_but_not_closing(subject.author)).to match_array([mentioned_issue])
     end
+
+    context 'when the project has an external issue tracker' do
+      before do
+        subject.project.team << [subject.author, :developer]
+        commit = double(:commit, safe_message: 'Fixes TEST-3')
+
+        create(:jira_service, project: subject.project)
+
+        allow(subject).to receive(:commits).and_return([commit])
+        allow(subject).to receive(:description).and_return('Is related to TEST-2 and TEST-3')
+        allow(subject.project).to receive(:default_branch).and_return(subject.target_branch)
+      end
+
+      it 'detects issues mentioned in description but not closed' do
+        expect(subject.issues_mentioned_but_not_closing(subject.author).map(&:to_s)).to match_array(['TEST-2'])
+      end
+    end
   end
 
   describe "#work_in_progress?" do