Add pipeline lists to GraphQL

This adds Keyset pagination to GraphQL lists. PoC for that is
pipelines on merge requests and projects.

When paginating a list, the base-64 encoded id of the ordering
field (in most cases the primary key) can be passed in the `before` or
`after` GraphQL argument.

4 files changed, 95 insertions, 0 deletions

diff --git a/lib/gitlab/graphql/connections.rb b/lib/gitlab/graphql/connections.rb
new file mode 100644
index 00000000000..2582ffeb2a8
--- /dev/null
+++ b/lib/gitlab/graphql/connections.rb
@@ -0,0 +1,12 @@
+module Gitlab
+  module Graphql
+    module Connections
+      def self.use(_schema)
+        GraphQL::Relay::BaseConnection.register_connection_implementation(
+          ActiveRecord::Relation,
+          Gitlab::Graphql::Connections::KeysetConnection
+        )
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/graphql/connections/keyset_connection.rb b/lib/gitlab/graphql/connections/keyset_connection.rb
new file mode 100644
index 00000000000..abee2afe144
--- /dev/null
+++ b/lib/gitlab/graphql/connections/keyset_connection.rb
@@ -0,0 +1,73 @@
+module Gitlab
+  module Graphql
+    module Connections
+      class KeysetConnection < GraphQL::Relay::BaseConnection
+        def cursor_from_node(node)
+          encode(node[order_field].to_s)
+        end
+
+        def sliced_nodes
+          @sliced_nodes ||=
+            begin
+              sliced = nodes
+
+              sliced = sliced.where(before_slice) if before.present?
+              sliced = sliced.where(after_slice) if after.present?
+
+              sliced
+            end
+        end
+
+        def paged_nodes
+          if first && last
+            raise Gitlab::Graphql::Errors::ArgumentError.new("Can only provide either `first` or `last`, not both")
+          end
+
+          if last
+            sliced_nodes.last(limit_value)
+          else
+            sliced_nodes.limit(limit_value)
+          end
+        end
+
+        private
+
+        def before_slice
+          if sort_direction == :asc
+            table[order_field].lt(decode(before))
+          else
+            table[order_field].gt(decode(before))
+          end
+        end
+
+        def after_slice
+          if sort_direction == :asc
+            table[order_field].gt(decode(after))
+          else
+            table[order_field].lt(decode(after))
+          end
+        end
+
+        def limit_value
+          @limit_value ||= [first, last, max_page_size].compact.min
+        end
+
+        def table
+          nodes.arel_table
+        end
+
+        def order_info
+          @order_info ||= nodes.order_values.first
+        end
+
+        def order_field
+          @order_field ||= order_info&.expr&.name || nodes.primary_key
+        end
+
+        def sort_direction
+          @order_direction ||= order_info&.direction || :desc
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/graphql/errors.rb b/lib/gitlab/graphql/errors.rb
new file mode 100644
index 00000000000..1d8e8307ab9
--- /dev/null
+++ b/lib/gitlab/graphql/errors.rb
@@ -0,0 +1,8 @@
+module Gitlab
+  module Graphql
+    module Errors
+      BaseError = Class.new(GraphQL::ExecutionError)
+      ArgumentError = Class.new(BaseError)
+    end
+  end
+end
diff --git a/lib/gitlab/graphql/present/instrumentation.rb b/lib/gitlab/graphql/present/instrumentation.rb
index 6f2b26c9676..f87fd147b15 100644
--- a/lib/gitlab/graphql/present/instrumentation.rb
+++ b/lib/gitlab/graphql/present/instrumentation.rb
@@ -3,6 +3,8 @@ module Gitlab
     module Present
       class Instrumentation
         def instrument(type, field)
+          return field unless field.metadata[:type_class]
+
           presented_in = field.metadata[:type_class].owner
           return field unless presented_in.respond_to?(:presenter_class)
           return field unless presented_in.presenter_class