Question: i'm starting now on Rails, i looked in the forum, but i didn't find anything that could solve my problem. 

Here it goes, I have a Category table, and it has only name for a column (there is no repetition in categories) so i would like name to be the primary key, then i have a Product table that has name, main\_photo, description and i would like to say that a product only has a category, do i need to add a column named category as a foreign key in products? 

A Category is suposed to have many products. 

Then in category models how do i say that name is the primary Key, and how can i do the correspondence between the suposed primary key name in categories and category in products?

Answer: 
Foreign key constraints in Active Record aren't used very often as the ideology behind Active Record says that this kind of logic should belong in the model and not in the database - the database is just a dumb store: <http://guides.rubyonrails.org/migrations.html#active-record-and-referential-integrity>.

The Rails way is to have an ID column on all tables including your Categories table, and in your Products table, have a column called Category\_ID. Notice that the table names are plurals.

Then in your model you define the relationships between the entities Product and Category. Read the article [*A Guide to Active Record Associations*](http://guides.rubyonrails.org/association_basics.html) and it will answer all your questions, especially sections 2.1, 2.2 and 3.3.
There are many valid reasons to have foreign keys in your database. See [Does Rails need database-level constraints?](https://stackoverflow.com/questions/2589509/does-rails-need-database-level-constraints/2589875#2589875)

I recommend [Foreigner](https://github.com/matthuhiggins/foreigner) if you want to easily add foreign keys to your Rails app.