Rails has added a new method
ActiveRecord::Relation#annotate
which allows annotating queries generated by
ActiveRecord::Relation instance SQL comments.
For eg:
prefix = "Na"
User.where("name LIKE ?", prefix).annotate("users with name starting with #{prefix}").to_sql
=> "SELECT \"users\".* FROM \"users\" WHERE (name LIKE 'Na') /* users with name starting with Na */"
When using annotate the query is generated
with the block-style SQL comment.
ActiveRecord::Relation#annotate
- We can pass multiple arguments to annotate which adds separate blocks.
User.where("name LIKE ?", prefix).annotate("select names", "users with name starting with #{prefix}").select(:name).to_sql
=> "SELECT \"users\".\"name\" FROM \"users\" WHERE (name LIKE 'Na') /* select names */ /* users with name starting with Na */"
- Multiple comments can also be added by chaining multiple
annotatemethod calls.
User.select(:name).annotate("select names").where("name LIKE ?", prefix).annotate("users with name starting with #{prefix}").to_sql
=> "SELECT \"users\".\"name\" FROM \"users\" WHERE (name LIKE 'Na') /* select names */ /* users with name starting with Na */"
annotateused in associations.
has_many :comments, -> { annotate("user comments") }
user.comments.to_sql
=> "SELECT \"comments\".* FROM \"comments\" WHERE \"comments\".\"user_id\" = 1 /* users comments */"
annotatein scopes.
scope :name_starts_with, ->(prefix) {
where("name like ?", prefix).annotate("user name starting with #{prefix}")
}
User.name_starts_with("Na")
=> "SELECT \"users\".* FROM \"users\" WHERE (name like 'Na') /* user name starting with Na */"
annotatein scoping
User.annotate("scoped").scoping do
User.all.to_sql
end
=> "SELECT \"users\".* FROM \"users\" /* scoped */"
This feature can be very useful when combined with Rails instrumentation or other ways of tracking and analyzing SQL queries generated at runtime in Rails applications.
Although there are other tools or gems that provide similar functionality
in some contexts,
they work on annotating all queries in request or job.
Using annotate we can annotate specific scopes or associations.