I've already posted it as a bug, but I got no response, so I open this discussion to better understand and explain my point of view.
In the current version of RubyMine (5.0.x) only birectional associations are show in model diagram, that is a relationship where we have something like
in many cases we have mono directional associations, where only belongs_to end is meaningful an we have no interest in the reverse path.
In RubyMine 4.x those kind of associations were displayed, now it seems that an association will be shown only if the "has_XXX" end is defined.
Many members of Rails community belive that the associations should be navigable in both ways, but in my opinion in many cases that is a modeling error, for example when a Value Object (in DDD meaning) is mapped as a table.
What do you think about that? I mean: is really a RubyMine bug that will be fixed or is a choice?
Personally I think the Model Diagram is the real killing feature of RubyMine, but in the current version is not really useful.