This patch has two main portions:
1. Add SQL comment support to Arel via Arel::Nodes::Comment.
2. Implement a Relation#annotate method on top of that.
== Adding SQL comment support
Adds a new Arel::Nodes::Comment node that represents an optional SQL
comment and teachers the relevant visitors how to handle it.
Comment nodes may be added to the basic CRUD statement nodes and set
through any of the four (Select|Insert|Update|Delete)Manager objects.
For example:
manager = Arel::UpdateManager.new
manager.table table
manager.comment("annotation")
manager.to_sql # UPDATE "users" /* annotation */
This new node type will be used by ActiveRecord::Relation to enable
query annotation via SQL comments.
== Implementing the Relation#annotate method
Implements `ActiveRecord::Relation#annotate`, which accepts a comment
string that will be appeneded to any queries generated by the relation.
Some examples:
relation = Post.where(id: 123).annotate("metadata string")
relation.first
# SELECT "posts".* FROM "posts" WHERE "posts"."id" = 123
# LIMIT 1 /* metadata string */
class Tag < ActiveRecord::Base
scope :foo_annotated, -> { annotate("foo") }
end
Tag.foo_annotated.annotate("bar").first
# SELECT "tags".* FROM "tags" LIMIT 1 /* foo */ /* bar */
Also wires up the plumbing so this works with `#update_all` and
`#delete_all` as well.
This feature is useful for instrumentation and general analysis of
queries generated at runtime.
The sentence "This is exactly the same as defining a class method ..."
is not true, so #33653 fixed it, but added changes repeat what is explained
a few lines below. We can remove this part since a user is able to get info
about the difference between scopes and class methods below.
Context https://github.com/rails/rails/pull/33653#discussion_r211105969.
Reverts #33653, 97feb4996b1c88f770101dfce6d4d3a6baf6bb33.
[ci skip] A regular expression was used to find a lot of missing Oxford
commas and add them. The regular expression was as follows.
", ([a-zA-Z0-9.\`:'\"]+ ){1,6}(or|and) "
Adds note to explain unexpected behavior described in issue #28561.
Developers may be surprised that the internal representation of similar
query conditions in Hash and Array formats in default_scope will yield
different results for model initialization.
Link: https://github.com/rails/rails/issues/28561
