Robert, thanks for the reply. validates_existence_of is a plugin -> http://github.com/bogdans83/validates_existence_of/. I'll try to make this as simple as possible. Here is what i'm trying to do:
A's has many B's and B's have 1 C. A = Users B = Books C = Libraries Hope this clarifies everything for you. Sorry about the mistakes in my first post. Thanks again for your help On Oct 1, 5:19 pm, Robert Walker <[email protected]> wrote: > mlittle wrote: > > Another newbie question by me... > > First, do yourself a favor and not start off by advertising your > "newbie" status. Trust that experienced programmers will know right away > where you are in your learning by reading your question. > > Before attempting to answer this question I would like some > confirmations. Read on... > > > I have this: > > > class User < ActiveRecord::Base > > ... > > has_one :book, :through => :library > > ... > > end > > Are you wanting to say here that a user can only check out one book at a > time from the library? > > > class Library < ActiveRecord::Base > > has_one :book, :dependent => :destroy > > end > > And a library can only ever contain one book? > > > class Library < ActiveRecord::Base > > belongs_to :library > > validates_existence_of :library > > end > > Did you mean to define Book here instead of reopening the Library class > and adding stuff to it? > > Assuming you meant Book it seems fine for a book to belong to a library. > > There is no validates_existence_of. Assuming you meant > validates_presence_of :library: > I'm even a little bit torn on this one, but I validate the presence of > the foreign key (:library_id in this case) rather than validate the > association. From my experience so far it seems to make for cleaner test > scripts and also does not directly depend on the actual association > object making model tests easier to manage. > > > I just don't know what my route will look like. > > Once you get your model actually right your routes will make sense. > > > Any help or suggestions? Are my models correct in how I used the has_one > > :through > > Keep this in mind as a general rule of thumb: The reason has_one exists > is for creating one-to-one associations. Thing A can have only one B and > thing B can only ever have one A. > > So why would you ever need one-to-one associations? Can't you just add > all the needed fields to one table? Technically the answer is "Yes you > can." However, there are some very useful and practical uses for > one-to-one associations. > > - Sometimes groups of fields, which actually are functionally depend on > the primary key of a row, are related in other ways and are good > candidate to be broken out into a separate table. > > - Often a table may contain a set of "optional" fields that don't need > to have values for every row in a table. These can be broken out to a > separate table for effeciency's sake and to clean up all those NULLs you > would see if there was only one table. > > - There are also cases where a table may contain a large character or > binary blob. Most of the time it's convenient to just use the wildcard > when selecting data: (select * from my_table;). If that table contained > a column of type (TEXT or BLOB). Then you're going to suffer a > significant performance hit. That could be fixed by always specifying > the list of columns with something like: (select Col1, Col2, Coln from > my_table;), but you could also use a one-to-many and put the TEXT or > BLOB in a separate table. Then (select * from my_table;) would only > fetch the foreign key (i.e. my_blob_id), which is a simple integer and > you won't suffer the performance hit. When you actually want to get the > large TEXT or BLOB you can either grab them individually thought the > association, or include them if you need to grab a list of objects: > @posts = Post.find(:all, :include => :post_narrative). > > So why has_one and not belongs_to? Well to answer this you need to > consider how a one-to-on association is built in Rails. It is really > just like a one-to-many association with a validation that only allows > one object to be associated on the "many" side. So think of has_one as a > "special" has_many than includes the proper validation (to ensure > one-to-one) and also provides the one object instead of an array > containing the one object. > > So which side gets the has_one? Well that's easy to answer as well: use > belongs_to on the model that contains the foreign key and use has_one on > the model that doesn't. You get to decide which model should get the > foreign key. Usually objects have a natural association that makes that > fairly easy. Say that a Person can only have one Residence (within this > design). You wouldn't think about a Person belonging to the Residence. > You would rather say the Residence belongs to the Person. So Residence > would get the foreign key, an hence the belongs_to :person. So then > Person gets the has_one: residence. > > I apologize for the long reply. Maybe this belongs_to :blog. > > -- > Posted viahttp://www.ruby-forum.com/. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en -~----------~----~----~----~------~----~------~--~---
