Question

I have a class structure like this:

class Parent < ActiveRecord::Base
    has_one :child
    accepts_nested_attributes_for :child
end

...

class Child < ActiveRecord::Base
    belongs_to :parent
    validates :text, :presence => true
end

In my mobile app (IOS + RestKit), I submit a HTTP POST request to create a new Parent object. This request has Content-Type application/json

The request looks like this on the wire:

{"parent":{"field1":"value1","child":{"text":"textvalue"},"duration":100}}

When Rails receives it, and tries to save it, I get the error:

ActiveRecord::AssociationTypeMismatch (Child(#2198796760) expected, got ActiveSupport::HashWithIndifferentAccess(#2158000780)):

Anyone know what’s going on?

Answer 1

To expand one what Sameer said, you need to send child_attributes instead of child. This means you can’t use the same mapping for pulling from the server and then pushing to it.

In RestKit you can specify a custom serialization that’s different from the original object mapping. Here’s an example that posts to a rails app, where Object has_many Images

//Map Images
RKManagedObjectMapping* imageMapping = [RKManagedObjectMapping mappingForEntityWithName:@"Image"];
imageMapping.setNilForMissingRelationships = YES;
imageMapping.primaryKeyAttribute = @"imageId";
[imageMapping mapKeyPathsToAttributes:@"id", @"imageId", @"is_thumbnail", @"isThumbnail", @"image_caption", @"imageCaption", @"image_data", @"imageData", nil];

//Serialize Images
RKManagedObjectMapping* imageSerialization = (RKManagedObjectMapping*)[imageMapping inverseMapping];
imageSerialization.rootKeyPath = @"image";
[imageSerialization removeMappingForKeyPath:@"imageId"];

//Map Objects
RKManagedObjectMapping* objectMapping = [RKManagedObjectMapping mappingForEntityWithName:@"Object"];
objectMapping.setNilForMissingRelationships = YES;
objectMapping.primaryKeyAttribute = @"objectId";
[objectMapping mapKeyPath:@"id" toAttribute:@"objectId"];
[objectMapping mapRelationship:@"images" withMapping:imageMapping];

//Serialize Objects
RKManagedObjectMapping* objectSerialization = (RKManagedObjectMapping*)[objectMapping inverseMapping];
objectSerialization.rootKeyPath = @"object";
[objectSerialization removeMappingForKeyPath:@"images"];
[objectSerialization removeMappingForKeyPath:@"objectId"];
[objectSerialization mapKeyPath:@"images" toRelationship:@"images_attributes" withMapping:imageSerialization];

[objectManager.mappingProvider setMapping:objectMapping forKeyPath:@"object"];
[objectManager.mappingProvider setSerializationMapping:objectSerialization forClass:[Object class]];

Note that it’s also important to remove the ID attributes when posting – that gave me no end of trouble, since it doesn’t seem like it should matter in a post, but it throws rails off.

For what it’s worth, I also had issues parsing the nested objects with rails, and had to change my controller to look like this:

def create
    images = params[:object].delete("images_attributes");
    @object = Object.new(params[:object])

    result = @object.save

    if images
      images.each do |image|
        image.delete(:id)
        @object.images.create(image)
      end
    end

    respond_to do |format|
      if result
        format.html { redirect_to(@object, :notice => 'Object was successfully created.') }
        format.json  { render :json => @object, :status => :created, :location => @object }
      else
        format.html { render :action => "new" }
        format.json  { render :json => @object.errors, :status => :unprocessable_entity }
      end
    end
  end

That could (and probably should) be moved into a before_create filter on the Object model.

I hope that helps in some way.