[xi]

People mean different things when they say relational model, so to clarify, by relational model I mean a model in which data is represented as sets of N-tuples of fixed structure, and queries are constructed using set-based operations such as filtering and Cartesian product.

Also, when I say path-based access, I mean access that follows predefined links between entities (in SQL, provided by FOREIGN KEY constraints). Those are well supported by object model and ORMs, as opposed to arbitrary joins, which aren't.

You don't need a relational model to represent one-to-many relations, in fact, an object model such as provided by many ORMs could represent them perfectly. In your first example, `figure.vertices` could be a list of vertices associated with a figure, and `vertex.figure` is the figure which owns the vertex. Similarly mutual object or list references could represent other singular or plural relationships. Though I agree it requires referential loops and cannot be expressed well in a pure hierarchical model such as in many novel no-sql databases.