# An Example of a Form With Many To Many Relationships **URL:** \ **Category:** Rails\ **Created:** [June 3, 2008, 12:35am UTC](https://www.ruby-forum.com/t/an-example-of-a-form-with-many-to-many-relationships/139337 "2008-06-03T00:35:29Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ror\_arch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ror\_arch](https://www.ruby-forum.com/u/ror_arch)\ **Post date:** [June 3, 2008, 12:35am UTC](https://www.ruby-forum.com/t/an-example-of-a-form-with-many-to-many-relationships/139337/1 "2008-06-03T00:35:29Z")
I wished to look at an example of a form that does the following: [table A] [table A\_B] [table B] a\_id A\_B\_id b\_id nameA a\_id nameB b\_id The above table basically allows for a many to many relationship as follows: A\<\<–\>A\_B\<–\>\>B Is there a tutorial or book or example out there that deals with creating a form as above? I am planning on using this fields\_for technique to embed forms of related models into one another. I ask because I just created a form where there are is a one to one relationship but I dont know how I will go about adding models with many to many relationships (as above) or an example of such a form would be great. I have run across multiple one to many relationship tutorials like the category assigned to a form etc. ---
**Author:** ![ror\_arch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ror\_arch](https://www.ruby-forum.com/u/ror_arch)\ **Post date:** [June 3, 2008, 1:23am UTC](https://www.ruby-forum.com/t/an-example-of-a-form-with-many-to-many-relationships/139337/2 "2008-06-03T01:23:56Z")
When you’re after a many-to-many relationship in Rails, you’ve basically got two options. Josh S. wrote a great article a while back on the two options with a sort of side-by-side comparison to help people wondering which method they should use. [http://blog.hasmanythrough.com/2006/4/20/many-to-many-dance-off](http://blog.hasmanythrough.com/2006/4/20/many-to-many-dance-off) IMO, a has\_and\_belongs\_to\_many association is perfect when I just need a straight many-to-many association and no other data regarding that association. has\_many :through comes in really handy when I do need to store some sort of data regarding an association. Eg. a many-to-many relationship between Magazine and Reader would be handled :through =\> :subscriptions. The benefit of this is that since Subscription is a separate model and not strictly a join table, I can store data in there like subscription\_date, renewal\_date, amount\_paid, etc. I said all of that to say this: the way you build your form will sort of hinge on which of these methods you decide to use to build your m-2-m relationship. Let me know what you decide. – Josh Ather S. wrote: > I wished to look at an example of a form that does the following: > > [table A] [table A\_B] [table B] > a\_id A\_B\_id b\_id > nameA a\_id nameB > b\_id > > The above table basically allows for a many to many relationship as > follows: > > A\<\<–\>A\_B\<–\>\>B > > Is there a tutorial or book or example out there that deals with > creating a form as above? I am planning on using this fields\_for > technique to embed forms of related models into one another. > > I ask because I just created a form where there are is a one to one > relationship but I dont know how I will go about adding models with many > to many relationships (as above) or an example of such a form would be > great. I have run across multiple one to many relationship tutorials > like the category assigned to a form etc. ---
**Author:** ![ror\_arch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ror\_arch](https://www.ruby-forum.com/u/ror_arch)\ **Post date:** [June 3, 2008, 1:30am UTC](https://www.ruby-forum.com/t/an-example-of-a-form-with-many-to-many-relationships/139337/3 "2008-06-03T01:30:30Z")
Joshua A. wrote: > When you’re after a many-to-many relationship in Rails, you’ve basically > got two options. Josh S. wrote a great article a while back on the > two options with a sort of side-by-side comparison to help people > wondering which method they should use. > > [has\_many :through - Many-to-many Dance-off!](http://blog.hasmanythrough.com/2006/4/20/many-to-many-dance-off) > > IMO, a has\_and\_belongs\_to\_many association is perfect when I just need a > straight many-to-many association and no other data regarding that > association. has\_many :through comes in really handy when I do need to > store some sort of data regarding an association. Eg. a many-to-many > relationship between Magazine and Reader would be handled :through =\> > :subscriptions. The benefit of this is that since Subscription is a > separate model and not strictly a join table, I can store data in there > like subscription\_date, renewal\_date, amount\_paid, etc. > > I said all of that to say this: the way you build your form will sort of > hinge on which of these methods you decide to use to build your m-2-m > relationship. > > Let me know what you decide. > > – Josh > > Ather S. wrote: > > > I wished to look at an example of a form that does the following: > > > > [table A] [table A\_B] [table B] > > a\_id A\_B\_id b\_id > > nameA a\_id nameB > > b\_id > > > > The above table basically allows for a many to many relationship as > > follows: > > > > A\<\<–\>A\_B\<–\>\>B > > > > Is there a tutorial or book or example out there that deals with > > creating a form as above? I am planning on using this fields\_for > > technique to embed forms of related models into one another. > > > > I ask because I just created a form where there are is a one to one > > relationship but I dont know how I will go about adding models with many > > to many relationships (as above) or an example of such a form would be > > great. I have run across multiple one to many relationship tutorials > > like the category assigned to a form etc. The association I have does have fields which require setting, meaning we do need to set some fields in the intermediate (middle) table. I am setting up my form such that it is centered around A’s MVC (or controller). Currently I am using this method outlined on page 492 of the Agile Web Dev with Rails 2006 edition (if anyone has a more recent e book I would love to have it please in exchange for other ROR, Java/J2ee etc. books). Thanks! ---
**Author:** ![ror\_arch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ror\_arch](https://www.ruby-forum.com/u/ror_arch)\ **Post date:** [June 4, 2008, 3:49pm UTC](https://www.ruby-forum.com/t/an-example-of-a-form-with-many-to-many-relationships/139337/4 "2008-06-04T15:49:01Z")
Ather S. wrote: > I wished to look at an example of a form that does the following: > > [table A] [table A\_B] [table B] > a\_id A\_B\_id b\_id > nameA a\_id nameB > b\_id > > The above table basically allows for a many to many relationship as > follows: > > A\<\<–\>A\_B\<–\>\>B > > Is there a tutorial or book or example out there that deals with > creating a form as above? I am planning on using this fields\_for > technique to embed forms of related models into one another. > > I ask because I just created a form where there are is a one to one > relationship but I dont know how I will go about adding models with many > to many relationships (as above) or an example of such a form would be > great. I have run across multiple one to many relationship tutorials > like the category assigned to a form etc. Ryan B. has a good recipe for this in the new Advanced Rails Recipes book. I’ve used a technique very similar to his, though mine uses a before\_validation callback to process the associated model attributes instead of overriding the attribute setter (that won’t make sense until you read his recipe). It’s on my list to blog about, but not high on the list. Get the book if you want to know what to do for now 🙂 – Josh S. [http://blog.hasmanythrough.com](http://blog.hasmanythrough.com) ---
**Author:** ![ror\_arch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ror\_arch](https://www.ruby-forum.com/u/ror_arch)\ **Post date:** [June 3, 2008, 1:55am UTC](https://www.ruby-forum.com/t/an-example-of-a-form-with-many-to-many-relationships/139337/5 "2008-06-03T01:55:24Z")
Well, the fact that you need additional fields in your join table sort of decides that you’re headed down the has\_many :through path. Ryan Bates screencast shows how to set up your models and migration to get started down that path in this episode. The first half is setting up for habtm, and even though you’re not looking to do that, I would suggest watching the whole thing. [http://media.railscasts.com/videos/047\_two\_many\_to\_many.mov](http://media.railscasts.com/videos/047_two_many_to_many.mov) – Josh Ather S. wrote: > Joshua A. wrote: > > > When you’re after a many-to-many relationship in Rails, you’ve basically > > got two options. Josh S. wrote a great article a while back on the > > two options with a sort of side-by-side comparison to help people > > wondering which method they should use. > > > > [has\_many :through - Many-to-many Dance-off!](http://blog.hasmanythrough.com/2006/4/20/many-to-many-dance-off) > > > > IMO, a has\_and\_belongs\_to\_many association is perfect when I just need a > > straight many-to-many association and no other data regarding that > > association. has\_many :through comes in really handy when I do need to > > store some sort of data regarding an association. Eg. a many-to-many > > relationship between Magazine and Reader would be handled :through =\> > > :subscriptions. The benefit of this is that since Subscription is a > > separate model and not strictly a join table, I can store data in there > > like subscription\_date, renewal\_date, amount\_paid, etc. > > > > I said all of that to say this: the way you build your form will sort of > > hinge on which of these methods you decide to use to build your m-2-m > > relationship. > > > > Let me know what you decide. > > > > – Josh > > > > Ather S. wrote: > > > > > I wished to look at an example of a form that does the following: > > > > > > [table A] [table A\_B] [table B] > > > a\_id A\_B\_id b\_id > > > nameA a\_id nameB > > > b\_id > > > > > > The above table basically allows for a many to many relationship as > > > follows: > > > > > > A\<\<–\>A\_B\<–\>\>B > > > > > > Is there a tutorial or book or example out there that deals with > > > creating a form as above? I am planning on using this fields\_for > > > technique to embed forms of related models into one another. > > > > > > I ask because I just created a form where there are is a one to one > > > relationship but I dont know how I will go about adding models with many > > > to many relationships (as above) or an example of such a form would be > > > great. I have run across multiple one to many relationship tutorials > > > like the category assigned to a form etc. > > The association I have does have fields which require setting, meaning > we do need to set some fields in the intermediate (middle) table. > > I am setting up my form such that it is centered around A’s MVC (or > controller). > > Currently I am using this method outlined on page 492 of the Agile Web > Dev with Rails 2006 edition (if anyone has a more recent e book I would > love to have it please in exchange for other ROR, Java/J2ee etc. books). > > Thanks!