# Needs some help with eager loading issue on a has\_many through relationship **URL:** \ **Category:** Rails\ **Created:** [December 2, 2008, 11:46pm UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688 "2008-12-02T23:46:15Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 2, 2008, 11:46pm UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/1 "2008-12-02T23:46:15Z")
I have the following: 1. A model called Student, which is an STI sub-class of Person 2. Student has the following associations has\_many :homerooms has\_many :classrooms, :through =\> :homerooms has\_one :current\_classroom, :through =\> :homerooms, :source =\> :classroom, :conditions =\> “classrooms.year = '” + $current\_school\_year + “’” 3. in the view, i have student.name + “(” + student.current\_classroom.grade + “)” I need to list all of the students, along with their classroom names (and teacher, but need to get over this hurdle first), so had the following Find in my index action of student\_controller ``` @students = Student.find(:all, :order => 'last_name, first_name', :include => :current_classroom) ``` But in reviewing the log, I see that people, homerooms, and classroom are all derived from separate queries. Student Load (0.000000) SELECT \* FROM `people` WHERE ( (`people`.`type` = ‘Student’ ) ) ORDER BY last\_name, first\_name Homeroom Load (0.002000) SELECT `homerooms`.\* FROM `homerooms` WHERE (`homerooms`.student\_id = 5122) Homeroom Columns (0.003000) SHOW FIELDS FROM `homerooms` Classroom Columns (0.003000) SHOW FIELDS FROM `classrooms` Classroom Load (0.000000) SELECT \* FROM `classrooms` WHERE (`classrooms`.`id` = 16) I have also tried the following ``` @students = Student.find(:all, :order => 'last_name, first_name', :include => {:homerooms => :classroom) ``` to no avail. In either case, the page displays fine, with students and classrooms. Any idea why Rails seems to be ignoring the :include? Any help would be greatly appreciated. Best, Tom ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 3, 2008, 2:55pm UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/2 "2008-12-03T14:55:39Z")
Fred, I appreciate your quick response. I am confused though. I thought the point of includes was to get all of the related objects in as few db calls as possible. From the log file, it is making individual calls for each classroom and each homeroom. So for every student, it finds that student’s homeroom with one call, then finds that homeroom’s classroom with another call. I supposed it will eventually cache all of the classrooms, but since homerooms is a join table, (one record for each student) it won’t be able to. What about has\_one :through in 2.1 is broken? I am running 2.1.2 Thanks for your help. Tom On Dec 3, 8:23 am, Frederick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 3, 2008, 2:24pm UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/3 "2008-12-03T14:24:30Z")
On Dec 2, 10:45 pm, hoenth [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Any idea why Rails seems to be ignoring the :include? It’s not - that’s the way it does includes now, one select per association type (unless it has to fallback to the old joins based code because conditions/order reference the included tables). In addition :include of has\_one through was broken in 2.1. Fred ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 3, 2008, 3:03pm UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/4 "2008-12-03T15:03:53Z")
On Dec 3, 1:55 pm, hoenth [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Fred, > > I appreciate your quick response. > > I am confused though. I thought the point of includes was to get all > of the related objects in as few db calls as possible. From the log > file, it is making individual calls for each classroom and each > homeroom. So for every student, it finds that student’s homeroom with > one call, then finds that homeroom’s classroom with another call. When you’re only loading one then the joins based stuff doesn’t make much sense. Were you to load 100 students the current code would load those students, then in query number 2 load all the homerooms and then the classrooms in the 3rd one. > I supposed it will eventually cache all of the classrooms, but since > homerooms is a join table, (one record for each student) it won’t be > able to. > > What about has\_one :through in 2.1 is broken? I am running 2.1.2 It was just screwed. Looks like it was backported to 2.1.2 though so you should be ok. Fred Fred ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 4, 2008, 4:13am UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/5 "2008-12-04T04:13:35Z")
hoenth wrote: > I thought it should only do 3 queries. > Can you post your ActiveRecord statement here? And if the :through is not working, can you replace :include =\> :students with :include =\> { :classes =\> :students }? Tell the includer the actual lookup path, so it improves the odds of eagerness? BTW if you use my inspect\_sql system (gem install assert\_efficient\_sql), you can print out the exact statements, and you can assert their properties, such as their count. – Phlip ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 4, 2008, 10:03am UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/6 "2008-12-04T10:03:04Z")
On Dec 4, 2:47 am, hoenth [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I am loading about 100 students. There is a single query to get them > “select \* from students…”. > > Then, it appears that it is doing an individual query for each > student, to find that students homeroom, then another for that > homeroom’s classroom. It is not loading all of the classrooms in a > single query, nor the homerooms. So for the 100 students, it does one > query for student, 100 queries for homerooms, and 100 queries for > classrooms (though some appear to be cached). I’d double check that your app is using rails 2.1.2. Even if rails -v reports 2.1.2 your app could still be using an earlier version if RAILS\_GEM\_VERSION is set or rails is frozen into vendor. It might also help to see slightly more of your code. Fred ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 4, 2008, 3:47am UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/7 "2008-12-04T03:47:43Z")
I am loading about 100 students. There is a single query to get them “select \* from students…”. Then, it appears that it is doing an individual query for each student, to find that students homeroom, then another for that homeroom’s classroom. It is not loading all of the classrooms in a single query, nor the homerooms. So for the 100 students, it does one query for student, 100 queries for homerooms, and 100 queries for classrooms (though some appear to be cached). I thought it should only do 3 queries. Thanks Tom On Dec 3, 9:02 am, Frederick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![hoenth](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/13edae/32.png) [@hoenth](https://www.ruby-forum.com/u/hoenth)\ **Post date:** [December 4, 2008, 4:18pm UTC](https://www.ruby-forum.com/t/needs-some-help-with-eager-loading-issue-on-a-has-many-through-relationship/154688/8 "2008-12-04T16:18:50Z")
Philip/Fred thank you for your thoughts. it never ceases to amaze me how some things manage to fix themselves. After looking into this for a couple of hours over the last two days, I reran the page today, and checking into the development log found that I am now down to 3 queries. The homerooms and classrooms query use large “where id in (…)” statements. I understood them to be inefficient compared to joins, but certainly better then individual calls. No idea why the change, but I appreciate your thoughts and help. best, Tom