Hello Guillaume and happy new year to you too. You are partly correct. At some point we will want to offer the ability to denormalize the information and copy the data of an entity when it is associated via a to-one association. This will be useful to load associated graphs in one lookup instead of two today. But as it is today, to-one associations are always IN_ENTITY as we always have the foreign key value in the document. That's exactly what is happening when you use IN_ENTITY for a to-many association in MongoDB, we store the association information (ie foreign key values) in the entity document. Frankly, I don't think copying the foreign key value of a to-one association is a separate document makes much sense: - FK are small - by definition, to-one only point to one value so the payload is minimal. Does that make sense? Emmanuel On Wed 2013-01-02 15:40, Guillaume SCHEIBEL wrote: