[hibernate-dev] Question about substituting IDs in Hibernate OGM

Thu Feb 12 16:13:58 EST 2015

Hi Gunnar,

I didn’t even think to look at the configuration, thanks ☺.   All of the inheritance strategies are supported in Datomic, but Join is probably going to be most commonly used (it certainly is in our schemata).

While it’s not a blocking concern, an important optimization would be to link all of the associations by temp ID and commit them in a big transaction, which I think will require some extension to OGM.  I suspect that you folks on OGM will have some good ideas about mapping JPA transactions to Datomic transactions.

I’d certainly be happy to work on getting some of this stuff into OGM if there are existing designs that I could help implement.  I’d also like to start contributing the project to OGM if it’s appropriate in order to get some more experienced eyes on it.

Any suggestions on how to proceed?

Thanks!

Josiah

From: gunnar.morling at googlemail.com [mailto:gunnar.morling at googlemail.com] On Behalf Of Gunnar Morling
Sent: Thursday, February 12, 2015 12:43 AM
To: Haswell, Josiah D
Cc: hibernate-dev at lists.jboss.org
Subject: Re: [hibernate-dev] Question about substituting IDs in Hibernate OGM

2015-02-12 1:37 GMT+01:00 Haswell, Josiah D <Josiah.Haswell at ca.com<mailto:Josiah.Haswell at ca.com>>:
Perfect.  That worked—thanks for the help!

Cool :)

I’ve made some pretty good progress—you can save entities and some associations, and retrieve some properties lazily.  Unfortunately, I’m having a bit of trouble with getting access to all of the type information I need.

In particular, say you have the hierarchy:

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)  (1)
@Entity
@Table(name = “persistable”)
Public class Persistable {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
Long id;

@Basic
String persistableName;

}

@Entity
@Table(“person_table”)
Public class Person extends Persistable {

@Basic
@Column(name = “first_name”)
String firstName;

@Basic
@Column
String lastName;

}

I need to generate the schema:

:persistable/persistableName

:Person/first_name
:Person/lastName

TABLE_PER_CLASS is meant to create a table for each type of the hierarchy with *all* its attributes. So actually you'd have to generate the following schema:

    :Persistable/persistableName

    :Person/persistableName
    :Person/first_name
    :Person/lastName

We also support SINGLE_TABLE which creates one table for all the types of the hierarchy, so the schema would have to be the following:

    :Persistable/persistableName
    :Persistable/first_name
    :Persistable/lastName
    :Persistable/DTYPE //denotes the entity type

Atm. we don't support JOINED (which may be what you had in mind).

And access it from a few contexts.

Schema Definition

When generating the schema, I need to be able to get the table name, which I was able to from the EntityPersisters available in SessionFactoryImpl.  From there, I needed to be able to get each property on each class, its type, and its alias (as defined by @Column), which I couldn’t figure out how to do.  I looked through both the EntityMetamodel and the ClassMetamodels on each persister, but it wasn’t clear how to get that.

Looking in the ClassMetadata, I found the property names, but that included all of the properties for all of the linear supertypes of the associated entity.
Looking at the EntityMetamodel, I found I could get all of the properties, but I couldn’t figure out how to filter out the properties declared in the current entity type.

I ended up scanning the annotations again to get the information I needed to generate the schema, which doesn’t seem like a good solution.

Schema definition is still a weak point in OGM atm., the grid dialects we began with tend to have no strong schema. There is the SchemaDefiner contract which you need to implement for this (and make known via YourDatastoreProvider#getSchemaDefinerType()). In YourSchemaDefiner#initializeSchema() you can obtain everything you need e.g. like so:

    @Override
    public void initializeSchema(Configuration configuration, SessionFactoryImplementor factory) {
        Iterator<PersistentClass> classMappings = configuration.getClassMappings();

        while ( classMappings.hasNext() ) {
            PersistentClass mapping = classMappings.next();
            Table table = mapping.getTable();

            Iterator<Column> columnIterator = table.getColumnIterator();
            while(columnIterator.hasNext()) {
                Column column = columnIterator.next();
                String columnName = column.getName();

                // do something with column...
            }
        }
    }

Note that this API stems straight from Hibernate ORM and is not exactly what we'd like to have for Hibernate OGM. So be prepared for changes in this area in future revisions (the entire SchemaDefiner contract is considered experimental atm., also Configuration will go away in Hibernate ORM in the future), but for now it should allow you to do the right thing. Definitely better than doing manual annotation scanning :)

Now, I’m running into the same problem retrieving a tuple:

public Tuple getTuple(EntityKey k, TupleContext c) {
final Long id = getOrThrowUnexpectedId(k);
final Entity entity = retrieveEntity(id);
return new Tuple(new LazyEntityBackedTupleSnapshot(entity, tupleContext));
}

public class LazyEntityBackedTupleSnapshot implements TupleSnapshot {
final Entity entity;
final TupleContext context;
//constructor, etc.

public Object get(String column) {
return entity.get(column);

}

}

Now, the problem is, say the entity that I retrieved was a Person.  If I call p.getPersistableName() , then entity.get(column) needs to be
Entity.get(“:persistable/persistableName”) to get the value.  I could search through the linear supertypes of the current type if I could get it.  I notice that the actual TupleContext that I’m getting has an OptionsContextImpl with an entity type that I can use to search for the appropriate entity type, but that’s private, and I don’t want to use reflection to get it.

See above, you should take all the column values from the Person table (or the Persistable table when working with SINGLE_TABLE). You can get the table name from the given entity key. From the tuple context btw. you can get the columns we actually need, so you don't need to fetch any unmapped columns from the datastore.

I’m sure I’m just missing some cool feature of OGM here, and couldn’t find it looking through the other implementations.  Any additional help would be fantastic.

Thanks!

No problem. Hope it helps,

--Gunnar

From: gunnar.morling at googlemail.com<mailto:gunnar.morling at googlemail.com> [mailto:gunnar.morling at googlemail.com<mailto:gunnar.morling at googlemail.com>] On Behalf Of Gunnar Morling
Sent: Wednesday, February 11, 2015 12:43 AM
To: Haswell, Josiah D
Cc: hibernate-dev at lists.jboss.org<mailto:hibernate-dev at lists.jboss.org>
Subject: Re: [hibernate-dev] Question about substituting IDs in Hibernate OGM

Hi Josiah,

It's great to hear that you are working an a backend for Hibernate OGM!

Regarding ids, it should work for you if they are mapped using the IDENTITY strategy:

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    Long id;

This causes the Hibernate ORM engine to read back the id value generated by the datastore upon insertion. To make it work, your GridDialect for OGM needs to implement the IdentityColumnAwareGridDialect facet [1]. You can check out the MongoDB dialect for an example.

If this works and this kind of id generation is the only one which is useful for Datomic (i.e. table/sequence strategies don't make any sense), you may validate mapped entities by means of a SchemaDefiner [2] (an experimental contract of OGM). An example is CouchDBSchemaDefiner.

Let us know how it goes or in case you run into any other issues.

--Gunnar

[1] https://github.com/hibernate/hibernate-ogm/blob/master/core/src/main/java/org/hibernate/ogm/dialect/identity/spi/IdentityColumnAwareGridDialect.java
[2] https://github.com/hibernate/hibernate-ogm/blob/master/couchdb/src/main/java/org/hibernate/ogm/datastore/couchdb/impl/CouchDBSchemaSchemaDefiner.java

2015-02-11 2:40 GMT+01:00 Haswell, Josiah D <Josiah.Haswell at ca.com<mailto:Josiah.Haswell at ca.com>>:
Hi folks,

I'm creating a Hibernate OGM implementation for Datomic, and I have a question about IDs.

Say I have the entity

@Entity
public class Person {
@Id
@GeneratedValue
Long id;

}

In Datomic, you have to assign a temporary ID before submitting the transaction.  Datomic will then return the actual persistence ID after the transaction has completed.  My question is, how can I get the actual ID back into the entity?

Thanks!

Josiah
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