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# 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
# 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)
@projects = projects
@limit = limit
@offset = offset
@filter = filter
end
# Returns an Array containing the events.
def to_a
return [] if current_page > MAX_PAGE
relation = if Gitlab::Database.join_lateral_supported?
relation_with_join_lateral
else
relation_without_join_lateral
end
relation.with_associations.to_a
end
private
# Returns the events relation to use when JOIN LATERAL is not supported.
#
# This relation simply gets all the events for all authorized projects, then
# limits that set.
def relation_without_join_lateral
events = filtered_events.in_projects(projects)
paginate_events(events)
end
# Returns the events relation to use when JOIN LATERAL is supported.
#
# This relation is built using JOIN LATERAL, producing faster queries than a
# regular LIMIT + OFFSET approach.
def relation_with_join_lateral
projects_for_lateral = projects.select(:id).to_sql
lateral = filtered_events
.limit(limit_for_join_lateral)
.where('events.project_id = projects_for_lateral.id')
.to_sql
# The outer query does not need to re-apply the filters since the JOIN
# LATERAL body already takes care of this.
outer = base_relation
.from("(#{projects_for_lateral}) projects_for_lateral")
.joins("JOIN LATERAL (#{lateral}) AS #{Event.table_name} ON true")
paginate_events(outer)
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
end
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