1 files changed, 103 insertions, 0 deletions
diff --git a/lib/gitlab/import_export/base_object_builder.rb b/lib/gitlab/import_export/base_object_builder.rb
new file mode 100644
index 00000000000..ec66b7a7a4f
--- /dev/null
+++ b/lib/gitlab/import_export/base_object_builder.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module ImportExport
+    # Base class for Group & Project Object Builders.
+    # This class is not intended to be used on its own but
+    # rather inherited from.
+    #
+    # Cache keeps 1000 entries at most, 1000 is chosen based on:
+    #    - one cache entry uses around 0.5K memory, 1000 items uses around 500K.
+    #      (leave some buffer it should be less than 1M). It is afforable cost for project import.
+    #    - for projects in Gitlab.com, it seems 1000 entries for labels/milestones is enough.
+    #      For example, gitlab has ~970 labels and 26 milestones.
+    LRU_CACHE_SIZE = 1000
+
+    class BaseObjectBuilder
+      def self.build(*args)
+        new(*args).find
+      end
+
+      def initialize(klass, attributes)
+        @klass = klass.ancestors.include?(Label) ? Label : klass
+        @attributes = attributes
+
+        if Gitlab::SafeRequestStore.active?
+          @lru_cache = cache_from_request_store
+          @cache_key = [klass, attributes]
+        end
+      end
+
+      def find
+        find_with_cache do
+          find_object || klass.create(prepare_attributes)
+        end
+      end
+
+      protected
+
+      def where_clauses
+        raise NotImplementedError
+      end
+
+      # attributes wrapped in a method to be
+      # adjusted in sub-class if needed
+      def prepare_attributes
+        attributes
+      end
+
+      private
+
+      attr_reader :klass, :attributes, :lru_cache, :cache_key
+
+      def find_with_cache
+        return yield unless lru_cache && cache_key
+
+        lru_cache[cache_key] ||= yield
+      end
+
+      def cache_from_request_store
+        Gitlab::SafeRequestStore[:lru_cache] ||= LruRedux::Cache.new(LRU_CACHE_SIZE)
+      end
+
+      def find_object
+        klass.where(where_clause).first
+      end
+
+      def where_clause
+        where_clauses.reduce(:and)
+      end
+
+      def table
+        @table ||= klass.arel_table
+      end
+
+      # Returns Arel clause:
+      # `"{table_name}"."{attrs.keys[0]}" = '{attrs.values[0]} AND {table_name}"."{attrs.keys[1]}" = '{attrs.values[1]}"`
+      # from the given Hash of attributes.
+      def attrs_to_arel(attrs)
+        attrs.map do |key, value|
+          table[key].eq(value)
+        end.reduce(:and)
+      end
+
+      # Returns Arel clause `"{table_name}"."title" = '{attributes['title']}'`
+      # if attributes has 'title key, otherwise `nil`.
+      def where_clause_for_title
+        attrs_to_arel(attributes.slice('title'))
+      end
+
+      # Returns Arel clause `"{table_name}"."description" = '{attributes['description']}'`
+      # if attributes has 'description key, otherwise `nil`.
+      def where_clause_for_description
+        attrs_to_arel(attributes.slice('description'))
+      end
+
+      # Returns Arel clause `"{table_name}"."created_at" = '{attributes['created_at']}'`
+      # if attributes has 'created_at key, otherwise `nil`.
+      def where_clause_for_created_at
+        attrs_to_arel(attributes.slice('created_at'))
+      end
+    end
+  end
+end