# Trying to Wrap My Head Around HM:t and HABTM w/ a Sample App **URL:** \ **Category:** Rails\ **Created:** [February 12, 2009, 9:11pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200 "2009-02-12T21:11:38Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 12, 2009, 9:11pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/1 "2009-02-12T21:11:38Z")
I am cheap and in an attempt to learn about rails & relationships I’m trying to create an application that my friends and I can use as a way to split the cost of items we buy @ Costco. Right now the models i have are: User - Various people involved Run - Represents Costco runs Item - Individual items purchased during a Costco run I was hoping that someone could point me in the right direction as to how i setup up the relationships… These are my models: Item.rb # An item is purchased on a run and it’s cost can be split between many Users. Because many Users can split the cost of an Item, it is owned by 1 or more Users. belongs\_to :run belongs\_to :user Run.rb # A run is an individual trip to costco. On a run many items are purchased. has\_many :items User.rb # A User can participate in many runs and during the run will be buying Items (or maybe buying a fraction of 1 item). has\_many :runs has\_many :items, :through =\> :participations #This is my area of question I realize this is incorrect as is, but could someone shed some light on how i should be thinking about a situation like this? Thanks for your time and for reading a big post like this 😉 ---
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 13, 2009, 4:37pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/2 "2009-02-13T16:37:59Z")
On Feb 13, 10:23 am, Marnen Laibow-Koser [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > On Feb 12, 3:11 pm, Josh T. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > wrote: > […] > > > User.rb > > > > # A User can participate in many runs and during the run will be buying > > > > Items (or maybe buying a fraction of 1 item). > > has\_many :runs > > has\_many :items, :through =\> :participations #This is my area of > > question > > Why do you need a :through at all here? has\_many :items should be > sufficient, if I understand the situation correctly. > […] Aack! Right after posting that, I realized something else. If a User can participate in many Runs, and a Run can have many Users associated with it, then you need class Run has\_and\_belongs\_to\_many :users end class User has\_and\_belongs\_to\_many :runs end That will create a join table (called runs\_users) associating Users with Runs. The join table will not be able to hold any additional information, and will not be associated with an ActiveRecord model type. If you need the join to hold additional information or be an ActiveRecord subclass, that’s what has\_many :through is for. In other words, has\_and\_belongs\_to\_many is kind of a special case of the simplest type of has\_many :through. Does that help? ## Best, Marnen Laibow-Koser removed\_email\_address@domain.invalid [http://www.marnen.org](http://www.marnen.org) ---
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 13, 2009, 5:49pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/3 "2009-02-13T17:49:15Z")
On Feb 13, 10:30 am, Marnen Laibow-Koser [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: […] > class Run > has\_and\_belongs\_to\_many :users > end > > class User > has\_and\_belongs\_to\_many :runs > end > > That will create a join table (called runs\_users) associating Users > with Runs. > […] Again with the posting too quickly…I made a silly error in this description. Of course this won’t _create_ a join table, but it will expect one in the database as I described. Sorry about my incoherence this morning. ## Best, Marnen Laibow-Koser marnen@marnenorg [http://www.marnen.org](http://www.marnen.org) ---
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 13, 2009, 4:25pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/4 "2009-02-13T16:25:33Z")
On Feb 12, 3:11 pm, Josh T. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: […] > belongs\_to :user > This looks fine. > Run.rb > > # A run is an individual trip to costco. On a run many items are > > purchased. > has\_many :items So does this. At some point, you might also need has\_many :users; it depends on what you’re doing. > User.rb > > # A User can participate in many runs and during the run will be buying > > Items (or maybe buying a fraction of 1 item). > has\_many :runs > has\_many :items, :through =\> :participations #This is my area of > question Why do you need a :through at all here? has\_many :items should be sufficient, if I understand the situation correctly. > I realize this is incorrect as is, but could someone shed some light on > how i should be thinking about a situation like this? > > Thanks for your time and for reading a big post like this 😉 You’re welcome! ## Best, Marnen Laibow-Koser removed\_email\_address@domain.invalid [http://www.marnen.org](http://www.marnen.org) ---
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 13, 2009, 6:52pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/5 "2009-02-13T18:52:08Z")
What about something like (not real code, just trying to convey the idea): Item (id, name) # an item is an item. given that you’ll probably # purchase the same item more than once, you could # generate an Item list as a shopping list for your # next Costco binge has\_many purchases Purchase (id, item\_id, run\_id, amount) # on a run, you bought some item that cost some # amount, and someone’s gonna pay belongs\_to item belongs\_to run has\_many shares Run (id, datetime?) # on this particular run, someone made some number of # purchases (User seems incidental to a run, important # only through its participation in shares) has\_many purchases Share (id, user\_id, purchase\_id, percent) # the cost of each purchase can be divided between # one or more users belongs\_to user belongs\_to purchase User(id, name) # each user can participate in any number of runs, and # would thereby be liable for some shares has\_many shares ---
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 15, 2009, 10:11pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/6 "2009-02-15T22:11:15Z")
Maybe this is a stupid follow up question, but could :through somehow apply to this scenario? ---
**Author:** ![spammy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/22d042/32.png) [@spammy](https://www.ruby-forum.com/u/spammy)\ **Post date:** [February 15, 2009, 10:02pm UTC](https://www.ruby-forum.com/t/trying-to-wrap-my-head-around-hm-t-and-habtm-w-a-sample-app/160200/7 "2009-02-15T22:02:48Z")
Hi Ar Chron and Marnen Laibow-Koser, Thanks a lot for responding. Both of your posts have helped me quite a bit. I think i need to go in the direction of Ar’s response though because it seems a bit more robust. Especially considering the introduction of the “Share” model as this will allow me to divide things up between Users using different %s per user. You do sometimes run into situations where people want to split things in unequal percents – ie 24 box of Snickers; user1 takes 6 snickers, user2 takes 6 snickers, and user3 gets 12 snickers and diabetes. I will give this a go and if it turns out half descent put it on github or something for other people who pride themselves on being cheap heheh 🙂