A POTENTIAL CONFUSION WHEN CONVERTING HABTM RELATIONSHIPS TO HAS_MANY :THROUGH.

As someone relatively new to Ruby on Rails, I managed to confuse myself on one of my Rails projects recently when I decided to follow some very logical advice in Obie Fernandez’s The Rails Way that said that has_and_belongs_to_many (habtm) relationships were essentially deprecated in Rails, and that has_many :through was the preferred way to handle many-to-many relationships.

With has_many :through, the join table becomes a full-fledged rails model, meaning that you have more flexibility to extend the functionality of that model as your application evolves in the future–a very sensible best practice. So, I decided to convert an existing relationship that I had specified as habtm to has_many :through. I began to see errors that initially confused me, along the lines of NameError: Uninitialized Constant …. I had misnamed my join model and table because I was still thinking from the habtm perspective instead of treating the new model as I would any other model in a has_many…belongs_to relationship.

To give a contrived example of what I did to cause this error, I had the following typical type of many-to-many relationship:

[clubs] –< [clubs_users] >– [users]

…which I initially coded in the model layer as:

class Club < ActiveRecord::Base # file name: “club.rb”, table name: “clubs” has_and_belongs_to_many :users end # implicit join table named: “clubs_users” class User < ActiveRecord::Base # file name: “user.rb”, table name: “users” has_and_belongs_to_many :clubs end

OK, lets review the obvious. In this scenario, ActiveRecord needs to work with the a join table that is not explicitly defined in the model layer. To automatically know about it, ActiveRecord looks for a table in the database that follows a naming convention: plural forms of the model names combined with an underscore (alphabetically), which in this case would be “clubs_users”.

In other words, Rails has to deduce the name of the underlying join table based on what information it has: the names of the two models defined on either side of the relationship. Once we use has_many :through, we have an explicit model definition for the join table, so Rails can directly know its table name based on the same naming conventions it uses to know that “clubs” is the database table for the “Club” model. The join model is just another model, following the same naming conventions as any other model.

So, while converting this relationship to a has_many :through definition, I momentarily forgot that the :through declaration is, after all, just another has_many option, and that for explicitly defined models, the underlying database table naming convention is to split any capitalized words with underscores and then (basically) pluralize the model name. Therefore, if I name the model representing the join table “ClubUser”, the underlying database table will be called “club_users” NOT “clubs_users” as I had initially expected.

class Club < ActiveRecord::Base # file name: “club.rb”, table name: “clubs” has_many :club_users has_many :users, :through => :club_users end class ClubUser < ActiveRecord::Base # file name: “club_user.rb”, table name: “club_users” belongs_to :user belongs_to :club end class User < ActiveRecord::Base # file name: “user.rb”, table name: “users” has_many :club_users has_many :clubs, :through => :club_users end

Luckily, Rails has fairly informative error messages and detailed logging by default, so I was able to see the error quickly and figure out why I was wrong. (Did I mention I am still primarily a Java developer, and I’m not used to things being convenient by default?). In retrospect, it seems an obvious, and frankly, pretty embarrassing error to make, but I recently saw others encountering the same issue, so I decided not to be so easily embarrassed and relate my encounter with this type of error and my understanding of how I created it. In the cases I have seen, people trip over this for the same reason I did: they were still thinking of the join relationship as following the naming rules of habtm, not a model in a has_many relationship.

Of course, if you are using a join model because you already know that you need to extend it with its own attributes, chances are that relationship has a recognizable name of its own. In this case, I probably would not have confused myself if I had decided to name my model “Membership” in the first place, then I would have known that the underlying table would be called “memberships”. In fact its hard to find documentation on has_many :through that doesn’t have such a named relationship. Again, here is how it would look:

class Club < ActiveRecord::Base # file name: “club.rb”, table name: “clubs” has_many :memberships has_many :users, :through => :memberships end class Membership < ActiveRecord::Base # file name: “membership.rb”, table name: “memberships” belongs_to :user belongs_to :club end class User < ActiveRecord::Base # file name: “user.rb”, table name: “users” has_many :memberships has_many :clubs, :through => :memberships end

But sometimes, you will have a join that doesn’t have a recognizable name like this, so something along the lines of “ClubUser” fits as well as any other name. I suspect that in this case, the possibility of confusing yourself may also arise. In these cases, or in the case where you just don’t like the default naming convention, you might consider using something like the following declaration in your model class to be more explicit about what is happening:

set_table_name “clubs_users”

This declaration is actually quite useful when you are trying to write a Rails application on top of a legacy database that doesn’t follow these conventions.

For a less contrived example of using has_many :through, see my earlier post on ternary associations in Rails. This is actually pretty close to some actual code I wrote that caused my initial confusion about has_many :through.

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