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Chapter 3. Active Record Relationships > has_one
3.3. has_one
The simplest database relationship is the one-to-one relationship. With Active Record, you can implement one-to-one relationships with either belongs_to or has_one. You decide whether to use belongs_to or has_one based on where the foreign key resides. The class associated to the table with the primary key uses belongs_to, and the other uses has_one. Figure 3-3 shows a has_one relationship.
Figure 3-3. In this one-to-one relationship, a Photo has_one File
