Add LabelsFinder

app/finders/labels_finder.rb

+class LabelsFinder
+  def initialize(current_user, params = {})
+    @current_user = current_user
+    @params = params
+  end
+
+  def execute
+    items = init_collection
+    items = with_title(items)
+    sort(items)
+  end
+
+  private
+
+  attr_reader :current_user, :params
+
+  def init_collection
+    label_ids = []
+    label_ids << Label.where(group_id: projects.where.not(group: nil).select(:namespace_id)).select(:id)
+    label_ids << Label.where(project_id: projects).select(:id)
+
+    union = Gitlab::SQL::Union.new(label_ids)
+
+    Label.where("labels.id IN (#{union.to_sql})")
+         .reorder(title: :asc)
+  end
+
+  def with_title(items)
+    items = items.where(title: title) if title.present?
+    items
+  end
+
+  def sort(items)
+    items.reorder(title: :asc)
+  end
+
+  def project_id
+    params[:project_id].presence
+  end
+
+  def title
+    params[:title].presence
+  end
+
+  def projects
+    return @projects if defined?(@projects)
+
+    if project_id
+      @projects = ProjectsFinder.new.execute(current_user)
+                                    .where(id: project_id)
+                                    .reorder(nil)
+    else
+      @projects = Project.none
+    end
+
+    @projects
+  end
+end