If a dataset contains one or more fields that together can be used to identify a resource in another dataset, or even another instance of the same resource, those fields taken together make up a “foreign key.” Foreign keys consist of the following:
The field or fields in the source dataset that make up the key
The target dataset that links to the key
The fields in the destination dataset that match up to the fields in the source dataset
When a foreign key exists in a dataset, the Observe user interface displays a link to follow the key relationship to the target dataset. You can also use foreign keys to
lookup values from the target dataset by browsing the relations.