Vertical Inheritance: Cannot Insert Record With Multiple Flattened Relationships

Copied and directly related to issue CAY-2122

When using the following DbEntities:

IV_OTHER (id, name)
IV_BASE (id, type, name)
IV_IMPL (id, attr1, attr2, other1_id, other2_id) (vertically inherits IV_BASE)

all fields are non-null

We end up with ObjEntities:
IvOther
IvBase (abstract)
IvImpl extends IvBase

When running the following code:

IvOther other1 = context.newObject(IvOther.class);
other1.setName("other1");

IvOther other2 = context.newObject(IvOther.class);
other2.setName("other2");

IvImpl impl = context.newObject(IvImpl.class);
impl.setName("Impl 1");
impl.setAttr1("attr1");
impl.setAttr2("attr2");
impl.setOther1(other1);
impl.setOther2(other2);

context.commitChanges();

Committing this errors in the database, because its trying to insert into the IV_IMPL table with a NULL value for OTHER2_ID

The original issue (and test) was referring to an Entity that only had 1 flattened relationship. I recently had to add another relationship to this Entity, and discovered the same bug pops up again. This time its because the new code does not handle merging the flattenedArcKeys if more than one exists per ObjectId.

I will upload the breaking test shortly.