ActiveRecord pluck to SQL - ruby-on-rails

Question

I know these two statements perform the same SQL:
Using select
User.select(:email)
# SELECT `users`.`email` FROM `users`
And using pluck
User.all.pluck(:email)
# SELECT `users`.`email` FROM `users`
Now I need to get the SQL statement derived from each method. Given that the select method returns an ActiveRecord::Relation, I can call the to_sql method. However, I cannot figure out how to get the SQL statement derived from a pluck operation on an ActiveRecord::Relation object, given that the result is an array.
Please, take into account that this is a simplification of the problem. The number of attributes plucked can be arbitrarily high.
Any help would be appreciated.

Answer 1

You cannot chain to_sql with pluck as it doesn't return ActiveRecord::relation. If you try to do, it throws an exception like so
NoMethodError: undefined method `to_sql' for [[""]]:Array
I cannot figure out how to get the SQL statement derived from a pluck
operation on an ActiveRecord::Relation object, given that the result
is an array.
Well, as #cschroed pointed out in the comments, they both(select and pluck) perform same SQL queries. The only difference is that pluck return an array instead of ActiveRecord::Relation. It doesn't matter how many attributes you are trying to pluck, the SQL statement will be same as select
Example:
User.select(:first_name,:email)
#=> SELECT "users"."first_name", "users"."email" FROM "users"
Same for pluck too
User.all.pluck(:first_name,:email)
#=> SELECT "users"."first_name", "users"."email" FROM "users"
So, you just need to take the SQL statement returned by the select and believe that it is the same for the pluck. That's it!

Answer 2

You could monkey-patch the ActiveRecord::LogSubscriber class and provide a singleton that would register any active record queries, even the ones that doesn't return ActiveRecord::Relation objects:
class QueriesRegister
include Singleton
def queries
#queries ||= []
end
def flush
#queries = []
end
end
module ActiveRecord
class LogSubscriber < ActiveSupport::LogSubscriber
def sql(event)
QueriesRegister.instance.queries << event.payload[:sql]
"#{event.payload[:name]} (#{event.duration}) #{event.payload[:sql]}"
end
end
end
Run you query:
User.all.pluck(:email)
Then, to retrieve the queries:
QueriesRegister.instance.queries