Relationships

One to One

• not frequently used, but important to know it's an option
• both tables can have only one record on either side of the relationship; each primary key value relates to only one (or no) record in the related table
• imagine a Library table has_one location, and a location belongs_to a specific library - that lets us look up solely by location, and see the connected library
• often, in situations like that, you can make the location an attribute of the library, but when a location has, for example, multiple fields (address 1, address 2, state, zip, etc.), it might make sense to create another table for addresses and set up a has_one relationship

One to Many

• the most common type of database relationship
• the primary key table contains only one record that relates to none, one, or many records in the related table
• an author has_many books, but a book belongs_to only one author

Many to Many

• also very frequent
• each record in both tables can relate to any number of records (or no records) in the other table.
• a book probably "has many" categories, and a category also probably "has many" books

Notes

Keep in mind, the belongs_to part always goes on the opposite side of the has_many or has_one. And the way it's stored is that the ID of the model that "has" something is stored in a field on the child, like "customer_id" or "author_id". In our example with authors and books, the Book model belongs_to the Author model, while the Author, as mentioned, has_many books.