Implementing Objectify Entity relation

0
votes

I m new to GAE and datastore. I m trying Objectify-version 5, to access datastore. I m very confused about the way a relationship is maintained across Entity with Objectify. I cant map the idea of relational schema to Objectify framework, for basic understanding. In relational DB (like the PK/FK stuff),its much easier to get. I find it very difficult to understand the relations (1- many,many-many) with Objectify.I read the objectify doc here, still no understanding, its not clear yet.For instance,I have an objectify Entity- Manager, another Entity- Employee. I want to maintain one-2-many relation between Manager-To-Employee. Questions are -

  1. How do I put a reference (like Foreign key in RDBMS) of Manager Entity in Employee Entity.
  2. After reference is set, how do I get all employees working for A Manager?
  3. How do I implement many-2-many relation for those Entities .
1
google-app-enginegoogle-cloud-datastoreobjectify
Objectify doesn't change the way you manage entity relationships so you should probably read the Datastore docs to familiarise yourself with the concepts before reading about Objectify. 1:n or n:n relationships are typically implemented using indexed Key properties or List<Key> properties, depending on your requirements. - tx802
thnk u, can u give me an example of one-many entity persistence, I mean how do I get the Manager entity reference in the Employee entity while inserting an Employee working for him, assuming Manager:employee=One:Many. Before persisting Employee entity, ofy().save().entity(emp).now(); how to get it a Manager reference. Thank u. - Tanvir

1 Answers

2
votes

as stated here:

How do I put a reference (like Foreign key in RDBMS) of Manager Entity in Employee Entity.

use Key<Manager> or Ref<Manager> . They are basically interchangable but Ref holds a get method to fetch the entity if needed.

After reference is set, how do I get all employees working for A Manager?

If you are using your "foreign key" approach, you just need to query the datastore for Employee entities with the field "manager" equal to a ref or Key to the manager ID, like it was any other value.

How do I implement many-2-many relation for those entities

You can store reference collections like List<Key<Employee>> or Set<Ref<Manager>