ModelsDatabasesRelationships.md

## Models, Databases, Relationships in Rails

#### What is the difference between a primary key and a foreign key? Where would we find a primary key? What would it be called by default? Where would we find a foreign key? What is the naming convention for a foreign key?

#### Write down one example of:
* a `one-to-one `relationship.
* a `one-to-many relationship`.
* a `many-to-many relationship`.