After specifying a relation, you have to specify keys for it. You do this by adding a Relationship Key element under the Entity Relation. The Relationship Key is a key-pair of a source entity and related entity columns. You can specify more than 1 key-pair if the relationship is multi-column.
|BaseEntityField||Specifies a column in the current entity.|
|RelatedEntityField||Specifies a column in the related entity, to which the BaseEntityColumn points to.|