Fail when issuable_meta_data is called on an unlimited collection

This method can be called with an array, or a relation:

1. Arrays always have a limited amount of values, so that's fine.
2. If the relation does not have a limit value applied, then we will load every
   single object in that collection, and prevent N+1 queries for the metadata
   for that. But that's wrong, because we should never call this without an
   explicit limit set. So we raise in that case, and this commit will see which
   specs fail.

The only failing specs here were the issues API specs, and the specs for
IssuableMetadata itself, and both have been addressed.

changelogs/unreleased/fix-issues-api-list-performance.yml

+---
+title: Speed up issues list APIs
+merge_request:
+author:
+type: performance

lib/api/issues.rb

@@ -68,7 +68,7 @@ module API
                          desc: 'Return issues for the given scope: `created-by-me`, `assigned-to-me` or `all`'
       end
       get do
-        issues = find_issues
+        issues = paginate(find_issues)
 
         options = {
           with: Entities::IssueBasic,
@@ -76,7 +76,7 @@ module API
           issuable_metadata: issuable_meta_data(issues, 'Issue')
         }
 
-        present paginate(issues), options
+        present issues, options
       end
     end
 
@@ -95,7 +95,7 @@ module API
       get ":id/issues" do
         group = find_group!(params[:id])
 
-        issues = find_issues(group_id: group.id)
+        issues = paginate(find_issues(group_id: group.id))
 
         options = {
           with: Entities::IssueBasic,
@@ -103,7 +103,7 @@ module API
           issuable_metadata: issuable_meta_data(issues, 'Issue')
         }
 
-        present paginate(issues), options
+        present issues, options
       end
     end
 
@@ -124,7 +124,7 @@ module API
       get ":id/issues" do
         project = find_project!(params[:id])
 
-        issues = find_issues(project_id: project.id)
+        issues = paginate(find_issues(project_id: project.id))
 
         options = {
           with: Entities::IssueBasic,
@@ -133,7 +133,7 @@ module API
           issuable_metadata: issuable_meta_data(issues, 'Issue')
         }
 
-        present paginate(issues), options
+        present issues, options
       end
 
       desc 'Get a single project issue' do

lib/gitlab/issuable_metadata.rb

 module Gitlab
   module IssuableMetadata
     def issuable_meta_data(issuable_collection, collection_type)
+      # ActiveRecord uses Object#extend for null relations.
+      if !(issuable_collection.singleton_class < ActiveRecord::NullRelation) &&
+          issuable_collection.respond_to?(:limit_value) &&
+          issuable_collection.limit_value.nil?
+
+        raise 'Collection must have a limit applied for preloading meta-data'
+      end
+
       # map has to be used here since using pluck or select will
       # throw an error when ordering issuables by priority which inserts
       # a new order into the collection.

spec/lib/gitlab/issuable_metadata_spec.rb

@@ -10,6 +10,10 @@ describe Gitlab::IssuableMetadata do
     expect(subject.issuable_meta_data(Issue.none, 'Issue')).to eq({})
   end
 
+  it 'raises an error when given a collection with no limit' do
+    expect { subject.issuable_meta_data(Issue.all, 'Issue') }.to raise_error(/must have a limit/)
+  end
+
   context 'issues' do
     let!(:issue) { create(:issue, author: user, project: project) }
     let!(:closed_issue) { create(:issue, state: :closed, author: user, project: project) }
@@ -19,7 +23,7 @@ describe Gitlab::IssuableMetadata do
     let!(:closing_issues) { create(:merge_requests_closing_issues, issue: issue, merge_request: merge_request) }
 
     it 'aggregates stats on issues' do
-      data = subject.issuable_meta_data(Issue.all, 'Issue')
+      data = subject.issuable_meta_data(Issue.all.limit(10), 'Issue')
 
       expect(data.count).to eq(2)
       expect(data[issue.id].upvotes).to eq(1)
@@ -42,7 +46,7 @@ describe Gitlab::IssuableMetadata do
     let!(:note) { create(:note_on_merge_request, author: user, project: project, noteable: merge_request, note: "a comment on a MR") }
 
     it 'aggregates stats on merge requests' do
-      data = subject.issuable_meta_data(MergeRequest.all, 'MergeRequest')
+      data = subject.issuable_meta_data(MergeRequest.all.limit(10), 'MergeRequest')
 
       expect(data.count).to eq(2)
       expect(data[merge_request.id].upvotes).to eq(1)