We need to store and retrieve entity association (e.g. @OneToOne, @ManyToOne, @ManytoMany). In JPA implementation, we need to implement table joining. Mongo supports SQL joining in aggregate command by:

• $lookup (https://www.mongodb.com/docs/atlas/atlas-stream-processing/sp-agg-lookup/)
• $unwind (https://www.mongodb.com/docs/manual/reference/operator/aggregation/unwind/)

This ticket is the very first step to explore the table joining. A POC implementation test case is at https://github.com/NathanQingyangXu/jpa-mongodb-mapping/blob/main/chameleon-core/src/test/java/org/hibernate/omm/join/SimpleJoinTests.java

We limit the scope to only @OneToOne and @ManyToOne with the following restrictions:

• no fetching customization as per https://docs.jboss.org/hibernate/orm/6.6/userguide/html_single/Hibernate_User_Guide.html#fetching (only consider eager loading of the toOne associated entities)
• should support multiple level of joining
• should work with @Embeddable @Struct.

Addressing the source code notes tagged with TODO-HIBERNATE-65 is in scope of this ticket.