Relationship Key
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.
attributes
| BaseEntityField | Specifies a column in the current entity. |
| RelatedEntityField | Specifies a column in the related entity which the BaseEntityColumn points to. |