Joining a related entity to a query

To build useful queries in PolicyCenter, you often must join related entities to the primary entity of the query. Typical reasons to create a join are to restrict the set of rows that the query returns or to retrieve additional information that the related entities contain. The query builder APIs support setting restrictions only on database-backed fields, which store their values directly in the database. Primary entities have many properties that are arrays, foreign keys, type lists, and derived fields. Related tables provide the database-backed fields for these types of properties. When you join related entities to a query, you can set restrictions on the related fields, which in turn restricts the primary entities in the result.