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# Foreign Keys and Object Relationships

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A foreign key is a combination of columns that reference a unique key, usually the primary key, in another table. Foreign keys can be any number of fields (similar to primary key), all of which are treated as a unit. A foreign key and the primary parent key it references must have the same number and type of fields.

Foreign keys represents relationships from a column or columns in one table to a column or columns in another table. For example, if every `Employee` has an attribute `address` that contains an instance of `Address` (which has its own descriptor and table), the one-to-one mapping for the `address` attribute would specify foreign key information to find an address for a particular `Employee`.

Refer to "Configuring Table and Field References (Foreign and Target Foreign Keys)" for more information.