Refactor Project.with_feature_available_for_user

This method used to use a UNION, which would lead to it performing the
same query twice; producing less than ideal performance. Further, in
certain cases ActiveRecord could get confused and mess up the variable
bindings, though it's not clear how/why exactly this happens.

Fortunately we can work around all of this by building some of the WHERE
conditions manually, allowing us to use a simple OR statement to get all
the data we want without any of the above problems.

app/models/project.rb

@@ -269,17 +269,29 @@ class Project < ActiveRecord::Base
   # project features may be "disabled", "internal" or "enabled". If "internal",
   # they are only available to team members. This scope returns projects where
   # the feature is either enabled, or internal with permission for the user.
+  #
+  # This method uses an optimised version of `with_feature_access_level` for
+  # logged in users to more efficiently get private projects with the given
+  # feature.
   def self.with_feature_available_for_user(feature, user)
-    return with_feature_enabled(feature) if user.try(:admin?)
+    visible = [nil, ProjectFeature::ENABLED]
 
-    unconditional = with_feature_access_level(feature, [nil, ProjectFeature::ENABLED])
+    if user&.admin?
-    return unconditional if user.nil?
+      with_feature_enabled(feature)
+    elsif user
+      column = ProjectFeature.quoted_access_level_column(feature)
 
-    conditional = with_feature_access_level(feature, ProjectFeature::PRIVATE)
+      authorized = user.project_authorizations.select(1).
-    authorized = user.authorized_projects.merge(conditional.reorder(nil))
+        where('project_authorizations.project_id = projects.id')
 
-    union = Gitlab::SQL::Union.new([unconditional.select(:id), authorized.select(:id)])
+      with_project_feature.
-    where(arel_table[:id].in(Arel::Nodes::SqlLiteral.new(union.to_sql)))
+        where("#{column} IN (?) OR (#{column} = ? AND EXISTS (?))",
+              visible,
+              ProjectFeature::PRIVATE,
+              authorized)
+    else
+      with_feature_access_level(feature, visible)
+    end
   end
 
   scope :active, -> { joins(:issues, :notes, :merge_requests).order('issues.created_at, notes.created_at, merge_requests.created_at DESC') }

app/models/project_feature.rb

@@ -27,6 +27,13 @@ class ProjectFeature < ActiveRecord::Base
 
       "#{feature}_access_level".to_sym
     end
+
+    def quoted_access_level_column(feature)
+      attribute = connection.quote_column_name(access_level_attribute(feature))
+      table = connection.quote_table_name(table_name)
+
+      "#{table}.#{attribute}"
+    end
   end
 
   # Default scopes force us to unscope here since a service may need to check

spec/models/project_feature_spec.rb

@@ -4,6 +4,18 @@ describe ProjectFeature do
   let(:project) { create(:empty_project) }
   let(:user) { create(:user) }
 
+  describe '.quoted_access_level_column' do
+    it 'returns the table name and quoted column name for a feature' do
+      expected = if Gitlab::Database.postgresql?
+                   '"project_features"."issues_access_level"'
+                 else
+                   '`project_features`.`issues_access_level`'
+                 end
+
+      expect(described_class.quoted_access_level_column(:issues)).to eq(expected)
+    end
+  end
+
   describe '#feature_available?' do
     let(:features) { %w(issues wiki builds merge_requests snippets repository) }