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# frozen_string_literal: true
# A collection of events to display in an event list.
#
# An EventCollection is meant to be used for displaying events to a user (e.g.
# in a controller), it's not suitable for building queries that are used for
# building other queries.
class EventCollection
include Gitlab::Utils::StrongMemoize
# To prevent users from putting too much pressure on the database by cycling
# through thousands of events we put a limit on the number of pages.
MAX_PAGE = 10
# projects - An ActiveRecord::Relation object that returns the projects for
# which to retrieve events.
# filter - An EventFilter instance to use for filtering events.
def initialize(projects, limit: 20, offset: 0, filter: nil, groups: nil)
@projects = projects
@limit = limit
@offset = offset
@filter = filter
@groups = groups
end
# Returns an Array containing the events.
def to_a
return [] if current_page > MAX_PAGE
relation = if groups
project_and_group_events
else
project_events
end
relation = paginate_events(relation)
relation.with_associations.to_a
end
def all_project_events
Event.from_union([project_events]).recent
end
private
def project_events
relation_with_join_lateral('project_id', projects)
end
def project_and_group_events
group_events = relation_with_join_lateral('group_id', groups)
Event.from_union([project_events, group_events]).recent
end
# This relation is built using JOIN LATERAL, producing faster queries than a
# regular LIMIT + OFFSET approach.
def relation_with_join_lateral(parent_column, parents)
parents_for_lateral = parents.select(:id).to_sql
lateral = filtered_events
.limit(limit_for_join_lateral)
.where("events.#{parent_column} = parents_for_lateral.id") # rubocop:disable GitlabSecurity/SqlInjection
.to_sql
# The outer query does not need to re-apply the filters since the JOIN
# LATERAL body already takes care of this.
base_relation
.from("(#{parents_for_lateral}) parents_for_lateral")
.joins("JOIN LATERAL (#{lateral}) AS #{Event.table_name} ON true")
end
def filtered_events
@filter ? @filter.apply_filter(base_relation) : base_relation
end
def paginate_events(events)
events.limit(@limit).offset(@offset)
end
def base_relation
# We want to have absolute control over the event queries being built, thus
# we're explicitly opting out of any default scopes that may be set.
Event.unscoped.recent
end
def limit_for_join_lateral
# Applying the OFFSET on the inside of a JOIN LATERAL leads to incorrect
# results. To work around this we need to increase the inner limit for every
# page.
#
# This means that on page 1 we use LIMIT 20, and an outer OFFSET of 0. On
# page 2 we use LIMIT 40 and an outer OFFSET of 20.
@limit + @offset
end
def current_page
(@offset / @limit) + 1
end
def projects
@projects.except(:order)
end
def groups
strong_memoize(:groups) do
groups.except(:order) if @groups
end
end
end
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