What about leading with something that sets the context, before defining the Mozart types?

This first sentence doesn’t actually say what “relations” are, and assumes that they are a fixed “knowable thing”, outside the scope of what Mozart defines them as. This is not necessarily the case (eg: the Rails docs refer to this same concept as “associations”).

eg: “Models in Mozart can be connected using *relations* which implement a basic set of associations, just like a relational database. There are two types of relations:”