It seems ActiveRecord's counter_cache feature can result in a counter cache being incremented twice. The scenario in which I am seeing this behavior is when I have two models that have a has_many :through relationship with each other through a join model (ie: Teacher has many Student through Classroom). When using the has_many :through generated methods to associate Teacher and Student directly (without manually creating the join record) the count goes up 2 times. Example: teacher.students << Student.create(name: "Bobby Joe") causes teacher.students_count to increment by 2.
Please help me find a solution that mitigates or eliminates this problem while allowing me to continue using built-in counter caching AND mass assignment through the has_many :through relation.
I have spent many hours looking for a solution and have extracted the problem out to a small test app which is the simplest failing example I could create. Any additional details needed to help me solve this should hopefully be below.
Example schema and models:
create_table :teachers do |t|
t.string :name
t.integer :students_count, default: 0
t.timestamps
end
class Teacher < ActiveRecord::Base
has_many :classrooms
has_many :students, :through => :classrooms
end
create_table :students do |t|
t.string :name
t.integer :teachers_count, default: 0
t.timestamps
end
class Student < ActiveRecord::Base
has_many :classrooms
has_many :teachers, :through => :classrooms
end
create_table :classrooms do |t|
t.references :teacher
t.references :student
t.timestamps
end
class Classroom < ActiveRecord::Base
belongs_to :student, :counter_cache => :teachers_count
belongs_to :teacher, :counter_cache => :students_count
end
Here is a short rails console session showing the steps taken and the fact that rails is executing two updates to teachers to increment students_count:
1.9.2-p290 :001 > t = Teacher.create(name: "Miss Nice")
SQL (9.7ms) INSERT INTO "teachers" ("created_at", "name", "students_count", "updated_at") VALUES (?, ?, ?, ?) [["created_at", Tue, 28 Feb 2012 03:31:53 UTC +00:00], ["name", "Miss Nice"], ["students_count", 0], ["updated_at", Tue, 28 Feb 2012 03:31:53 UTC +00:00]]
=> #<Teacher id: 1, name: "Miss Nice", students_count: 0, created_at: "2012-02-28 03:31:53", updated_at: "2012-02-28 03:31:53">
1.9.2-p290 :002 > t.students << Student.new(name: "Mary Ann")
SQL (0.3ms) INSERT INTO "students" ("created_at", "name", "teachers_count", "updated_at") VALUES (?, ?, ?, ?) [["created_at", Tue, 28 Feb 2012 03:32:12 UTC +00:00], ["name", "Mary Ann"], ["teachers_count", 0], ["updated_at", Tue, 28 Feb 2012 03:32:12 UTC +00:00]]
SQL (0.3ms) INSERT INTO "classrooms" ("created_at", "student_id", "teacher_id", "updated_at") VALUES (?, ?, ?, ?) [["created_at", Tue, 28 Feb 2012 03:32:12 UTC +00:00], ["student_id", 1], ["teacher_id", 1], ["updated_at", Tue, 28 Feb 2012 03:32:12 UTC +00:00]]
SQL (0.2ms) UPDATE "students" SET "teachers_count" = COALESCE("teachers_count", 0) + 1 WHERE "students"."id" = 1
Teacher Load (0.1ms) SELECT "teachers".* FROM "teachers" WHERE "teachers"."id" = 1 LIMIT 1
SQL (0.1ms) UPDATE "teachers" SET "students_count" = COALESCE("students_count", 0) + 1 WHERE "teachers"."id" = 1
SQL (0.0ms) UPDATE "teachers" SET "students_count" = COALESCE("students_count", 0) + 1 WHERE "teachers"."id" = 1
Student Load (0.2ms) SELECT "students".* FROM "students" INNER JOIN "classrooms" ON "students"."id" = "classrooms"."student_id" WHERE "classrooms"."teacher_id" = 1
=> [#<Student id: 1, name: "Mary Ann", teachers_count: 1, created_at: "2012-02-28 03:32:12", updated_at: "2012-02-28 03:32:12">]
I've put the entire test app on github if anyone would like to look closer (https://github.com/carlzulauf/test_app). I also created a unit test which demonstrates the issue and fails to pass (https://github.com/carlzulauf/test_app/blob/master/test/unit/classroom_test.rb)
