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# frozen_string_literal: true
# Keyset::Connection provides cursor based pagination, to avoid using OFFSET.
# It basically sorts / filters using WHERE sorting_value > cursor.
# We do this for performance reasons (https://gitlab.com/gitlab-org/gitlab-foss/issues/45756),
# as well as for having stable pagination
# https://graphql-ruby.org/pro/cursors.html#whats-the-difference
# https://coderwall.com/p/lkcaag/pagination-you-re-probably-doing-it-wrong
#
# It currently supports sorting on two columns, but the last column must
# be the primary key. If it's not already included, an order on the
# primary key will be added automatically, like `order(id: :desc)`
#
# Issue.order(created_at: :asc).order(:id)
# Issue.order(due_date: :asc)
#
# You can also use `Gitlab::Database.nulls_last_order`:
#
# Issue.reorder(::Gitlab::Database.nulls_last_order('due_date', 'DESC'))
#
# It will tolerate non-attribute ordering, but only attributes determine the cursor.
# For example, this is legitimate:
#
# Issue.order('issues.due_date IS NULL').order(due_date: :asc).order(:id)
#
# but anything more complex has a chance of not working.
#
module Gitlab
module Graphql
module Pagination
module Keyset
class Connection < GraphQL::Pagination::ActiveRecordRelationConnection
include Gitlab::Utils::StrongMemoize
def cursor_for(node)
encoded_json_from_ordering(node)
end
def sliced_nodes
@sliced_nodes ||=
begin
OrderInfo.validate_ordering(ordered_items, order_list)
sliced = ordered_items
sliced = slice_nodes(sliced, before, :before) if before.present?
sliced = slice_nodes(sliced, after, :after) if after.present?
sliced
end
end
def nodes
# These are the nodes that will be loaded into memory for rendering
# So we're ok loading them into memory here as that's bound to happen
# anyway. Having them ready means we can modify the result while
# rendering the fields.
@nodes ||= load_paged_nodes.to_a
end
private
def load_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) # rubocop: disable CodeReuse/ActiveRecord
end
end
# rubocop: disable CodeReuse/ActiveRecord
def slice_nodes(sliced, encoded_cursor, before_or_after)
decoded_cursor = ordering_from_encoded_json(encoded_cursor)
builder = QueryBuilder.new(arel_table, order_list, decoded_cursor, before_or_after)
ordering = builder.conditions
sliced.where(*ordering).where.not(id: decoded_cursor['id'])
end
# rubocop: enable CodeReuse/ActiveRecord
def limit_value
@limit_value ||= [first, last, max_page_size].compact.min
end
def ordered_items
strong_memoize(:ordered_items) do
unless items.primary_key.present?
raise ArgumentError.new('Relation must have a primary key')
end
list = OrderInfo.build_order_list(items)
# ensure there is a primary key ordering
if list&.last&.attribute_name != items.primary_key
items.order(arel_table[items.primary_key].desc) # rubocop: disable CodeReuse/ActiveRecord
else
items
end
end
end
def order_list
strong_memoize(:order_list) do
OrderInfo.build_order_list(ordered_items)
end
end
def arel_table
items.arel_table
end
# Storing the current order values in the cursor allows us to
# make an intelligent decision on handling NULL values.
# Otherwise we would either need to fetch the record first,
# or fetch it in the SQL, significantly complicating it.
def encoded_json_from_ordering(node)
ordering = { 'id' => node[:id].to_s }
order_list.each do |field|
field_name = field.attribute_name
ordering[field_name] = node[field_name].to_s
end
encode(ordering.to_json)
end
def ordering_from_encoded_json(cursor)
Gitlab::Json.parse(decode(cursor))
rescue JSON::ParserError
raise Gitlab::Graphql::Errors::ArgumentError, "Please provide a valid cursor"
end
end
end
end
end
end
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