Avoid regenerating the ref path for the environment

Closes #39752

app/models/environment.rb

@@ -110,7 +110,7 @@ class Environment < ActiveRecord::Base
   end
 
   def ref_path
-    "refs/#{Repository::REF_ENVIRONMENTS}/#{generate_slug}"
+    "refs/#{Repository::REF_ENVIRONMENTS}/#{slug}"
   end
 
   def formatted_external_url
@@ -164,6 +164,10 @@ class Environment < ActiveRecord::Base
     end
   end
 
+  def slug
+    super.presence || generate_slug
+  end
+
   # An environment name is not necessarily suitable for use in URLs, DNS
   # or other third-party contexts, so provide a slugified version. A slug has
   # the following properties:

changelogs/unreleased/sh-fix-environment-slug-generation.yml

+---
+title: Avoid regenerating the ref path for the environment
+merge_request:
+author:
+type: fixed

spec/models/environment_spec.rb

@@ -547,6 +547,15 @@ describe Environment do
 
       expect(environment.slug).to eq(original_slug)
     end
+
+    it "regenerates the slug if nil" do
+      environment = build(:environment, slug: nil)
+
+      new_slug = environment.slug
+
+      expect(new_slug).not_to be_nil
+      expect(environment.slug).to eq(new_slug)
+    end
   end
 
   describe '#generate_slug' do
@@ -583,6 +592,12 @@ describe Environment do
     it 'returns a path that uses the slug and does not have spaces' do
       expect(environment.ref_path).to start_with('refs/environments/staging-review-1-')
     end
+
+    it "doesn't change when the slug is nil initially" do
+      environment.slug = nil
+
+      expect(environment.ref_path).to eq(environment.ref_path)
+    end
   end
 
   describe '#external_url_for' do