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-rw-r--r--activerecord/lib/active_record/relation/calculations.rb8
1 files changed, 8 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 42c9881b48..56cf9bcd27 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -19,6 +19,14 @@ module ActiveRecord
#
# Person.group(:city).count
# # => { 'Rome' => 5, 'Paris' => 3 }
+ #
+ # If +count+ is used with +select+, it will count the selected columns:
+ #
+ # Person.select(:age).count
+ # # => counts the number of different age values
+ #
+ # Note: not all valid +select+ expressions are valid +count+ expressions. The specifics differ
+ # between databases. In invalid cases, an error from the databsae is thrown.
def count(column_name = nil, options = {})
# TODO: Remove options argument as soon we remove support to
# activerecord-deprecated_finders.