Remove redundant WHERE from event queries

The default scope in Event would add a "WHERE author_id IS NOT NULL"
clause to every query. Now that "events.author_id" has a NOT NULL clause
set this filter is redundant and we can safely remove it.

In this commit we also add a validation on events.author_id (previously
only defined in PushEvent) just in case something tries to create data
without an author ID. This way we can present a nicer error message
compared to PostgreSQL's foreign key error messages.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38129

app/models/event.rb

 class Event < ActiveRecord::Base
   include Sortable
   include IgnorableColumn
-  default_scope { reorder(nil).where.not(author_id: nil) }
+  default_scope { reorder(nil) }
   CREATED   = 1
   UPDATED   = 2
@@ -77,6 +77,12 @@ class Event < ActiveRecord::Base
   scope :for_milestone_id, ->(milestone_id) { where(target_type: "Milestone", target_id: milestone_id) }
+  # Authors are required as they're used to display who pushed data.
+  #
+  # We're just validating the presence of the ID here as foreign key constraints
+  # should ensure the ID points to a valid user.
+  validates :author_id, presence: true
+
   self.inheritance_column = 'action'
   # "data" will be removed in 10.0 but it may be possible that JOINs happen that

app/models/push_event.rb

@@ -3,12 +3,6 @@ class PushEvent < Event
   # different "action" value.
   validate :validate_push_action
-  # Authors are required as they're used to display who pushed data.
-  #
-  # We're just validating the presence of the ID here as foreign key constraints
-  # should ensure the ID points to a valid user.
-  validates :author_id, presence: true
-
   # The project is required to build links to commits, commit ranges, etc.
   #
   # We're just validating the presence of the ID here as foreign key constraints

changelogs/unreleased/events-redundant-where.yml

+---
+title: Remove redundant WHERE from event queries
+merge_request:
+author:
+type: other