Optimise query for previous_deployment

- No need to search via environment.name as environments must have
unique name per project.

app/models/deployment.rb

@@ -243,9 +243,9 @@ class Deployment < ApplicationRecord
 
   def previous_deployment
     @previous_deployment ||=
-      project.deployments.joins(:environment)
-      .where(environments: { name: self.environment.name }, ref: self.ref)
-      .where.not(id: self.id)
+      self.class.for_environment(environment_id)
+      .where(ref: ref)
+      .where.not(id: id)
       .order(id: :desc)
       .take
   end

spec/models/deployment_spec.rb

@@ -574,7 +574,7 @@ RSpec.describe Deployment do
 
   describe '#previous_deployment' do
     it 'returns the previous deployment' do
-      deploy1 = create(:deployment)
+      deploy1 = create(:deployment, :success)
       deploy2 = create(
         :deployment,
         project: deploy1.project,
@@ -583,6 +583,18 @@ RSpec.describe Deployment do
 
       expect(deploy2.previous_deployment).to eq(deploy1)
     end
+
+    it 'returns nothing if the refs do not match' do
+      deploy1 = create(:deployment, :success)
+      deploy2 = create(
+        :deployment,
+        :review_app,
+        project: deploy1.project,
+        environment: deploy1.environment
+      )
+
+      expect(deploy2.previous_deployment).to be_nil
+    end
   end
 
   describe '#link_merge_requests' do