Connecting Across 1-to-Many (Not Many-to-1) Relationships
Dave Thompson
Blake et al - superb framework and superb documentation. Thanks. The
following question on relationship connections is similar to one asked
previously by Gregory Combs, but sadly that didn't have a conclusive
answer. So here goes:
Sku and OrderComponent are two entities connected by a one to many
relationship. The relationship exists in Core Data as usual, and is
duplicated by an explicit foreign key property from OrderComponent to
Sku, as follows:
Sku:
------
SkuID (PK)
OrderComponent:
------------------------
OrderComponentID (PK)
SkuID (FK)
The iOS app receives two distinct JSON payloads, one containing Skus
and the other containing OrderComponents. The Sku payload contains
only SkuIDs (as well as other attributes which are outside the scope
of this question). The OrderComponent payload contains both
OrderComponentIDs and the corresponding SkuIDs for linking to. There
is no nested data in either payload.
I have two scenarios:
(1) The Sku payload arrives first, followed by the OrderComponent
payload.
(2) The OrderComponent payload arrives first, followed by the Sku
payload.
In both scenarios, I would like to connect the Skus / OrderComponents
to any existing OrderComponents / Skus, based on the SkuID which is
present in both payloads. In scenario (1), I'm able to do this
successfully using the following code:
[orderComponentObjectMapping hasOne:@"sku"
withMapping:skuObjectMapping];
[orderComponentObjectMapping connectRelationship:@"sku"
withObjectForPrimaryKeyAttribute:@"skuID"];
However, in scenario (2) I'm not able to make the connection, nor am I
clear on how or whether it can be done. In scenario (1), it's possible
because there is a foreign key on OrderComponent called skuID, which
points to the primary key on Sku. In scenario (2), this same mechanism
isn't applicable, as there is no foreign key from Sku back to
OrderComponent. Due to the nature of the relationship (1-to-many
rather than many-to-1), it is also not possible to add a foreign key.
Am I missing something trivial, or should I write custom code to
handle this scenario within didLoadObjects or similar?
Thanks to everyone for your great work on RestKit,
Dave
Shane Zatezalo
- an OrderComponent being created within the Core Data Store that has a NULL SKU
- later that SKU being created, but it is not linked, in Core Data, to any orderComponents.
You have a OrderComponent which hasOne SKU.
You have a SKU which hasMany OrderComponents.
If your mapping is:
> [orderComponentObjectMapping hasOne:@"sku"
> withMapping:skuObjectMapping];
> [orderComponentObjectMapping connectRelationship:@"sku"
> withObjectForPrimaryKeyAttribute:@"skuID"];
And your Core Data Entity has OrderComponent.SKU -> SKU (singular) and
SKU.OrderComponents => OrderComponent.SKU (has many)
When you create an orderComponent object via Restkit, it'll create the object in the Core Data Store with orderComponent pointing to a NULL SKU. Though that orderComponent would intact have orderComponent.SkuID as a valid FK to a nonexistent SKU object.
It seems like if you are going to do #2, you have to do a secondary check (after all orderComponents are created) for any orderComponents w/ a null SKU. Then see if a SKU with that SkuID exists. And if so, set orderComponent.sku = [Sku objectWithId: SkuID]
to create that relationship "afterward" by hand. If Restkit has a way to handle such automatically (and it may) I've not come across it. I think there's a branch or someone who's done some work with "syncing", you might want to search the google group. Their work may include something that would catch such a relationship setup and update it for you.
Shane
Dave Thompson
Thanks for the prompt reply. I've taken a look through the mails on
syncing - there's definitely some useful background info there.
I'll push ahead with a little custom code within didLoadObjects,
setting orderComponent.sku = [Sku objectWithId: SkuID] where
applicable, as you suggest. It's good to know I wasn't missing
anything obvious before going down that route.
Thanks again,
Dave
Bob Spryn
https://github.com/RestKit/RestKit/commit/789b0b99eb95c1c8f9f8bbf26638bf3a64d7a852
