Estimate: 2 weeks.

After we finished BsonArray embedding, a natural next step is to implement some basic array SQL function to use in WHERE clause, including (see https://docs.jboss.org/hibernate/orm/6.6/userguide/html_single/Hibernate_User_Guide.html#hql-functions-arrays):

• array_contains
• array_includes

So for the following HQL:

select * from Book where array_contains(tags, "classic")

for the following Book entity:

@Entity
@Table(name = "books")
class Book {       
    @Id int id;
    String title;
    List<String> tags;
} 

The difference between the two functions is the former test a single value (like the example above), whereas the latter tests on another array to see whether it is a subset.

There are many array SQL functions but the above two are the most basic ones and should satisfy lots of requirements.

Hibernate extension points:

• initializeFunctionRegistry method in Dialect (we could register custom SQL function in Dialect, just as we can register custom types as we have done for ObjectId type; see relevant Hibernate user guide: https://docs.jboss.org/hibernate/orm/6.6/userguide/html_single/Hibernate_User_Guide.html#hql-user-defined-functions)

PoC PR:

• commit: https://github.com/NathanQingyangXu/jpa-mongodb-mapping/commit/964d00a33c24430d032c6f6978287948735bfbd0 and https://github.com/NathanQingyangXu/jpa-mongodb-mapping/commit/d12f1f41199194aa8283fcf984a4c4f9263af0b2
• Test case: 
  org.hibernate.omm.type.aray.SimpleArrayFieldTests#test_query_mongo_embedded_array_field()