[9rx]

> How many ORMs support composite primary keys and non-binary relationships?

Theoretically all. However, often ORM and query building are combined into a unified framework that links them together into a higher order abstraction. You may know this type of thing by names like the active record pattern or the data mapper pattern, with concrete implementations like ActiveRecord and Entity Framework. Such frameworks often make it difficult to use the ORM component in isolation (and even where that isn't the case, doing so largely defeats the purpose of using the framework anyway), and as such you become constrained by the limits of the query builder. I suspect that is what you are really thinking of.

[reaanb2]

EF is limited to binary relationships: https://learn.microsoft.com/en-us/ef/core/modeling/relations.... When entities are mapped to rows and relationships to foreign key constraints, it results in a network data model interpretation of SQL databases. The relational model is better conceptualized from a fact-oriented modeling perspective, in which entities map to values, domains to FK constraints, relationships to the association of two or more entity sets in a table, and rows represent propositions about entities rather than entities themselves. However, non-binary relationships don't easily translate into navigational data access (cf. Bachman's paper "The programmer as navigator" about the network data model) and a logical view is difficult for the non-mathematical programmer, while structs and pointers are naïvely easy to understand, that's why ORMs remain popular. The impedance mismatch is less about mapping flat to rich data structures than about the network data model vs fact-oriented perspectives and row-at-a-time vs batch data access.

[9rx]

> EF is limited to binary relationships

But due to the constraints of its querying abilities, not the ORM aspect. ORM is only concerned with the data once it is materialized. How you materialize that data, be it the product of a network relationship, or a fact-oriented model, is immaterial. Remember, EF isn't just an ORM toolkit. It is an ORM toolkit plus a number of other features. But since we're only talking about ORM those other features don't relate to the discussion.

> while structs and pointers are naïvely easy to understand, that's why ORMs remain popular.

And, well, because it is an all-round good idea to turn externally-sourced data models into internal data models anyway. It makes life a hell a lot easier as things start to change over time. Even if you were to receive serialized data that is shaped exactly in the structure you want it at a given moment in time, you still would want to go through a mapping process to provide the clean break from someone else's data model and your own. This isn't limited to SQL database tables. You'd do the same if you received, say, JSON from a web service.