Differentiating between domain, model, and entity with respect to MVC

Question

Can someone explain these 3 concepts and the differences between them with respect to an MVC framework along with an example. To me these appear almost equivalent, and it seems they are used interchangeably in some articles and not in others.

Answer 1

The terms you are confused about are: "domain objects", "domain entities" and "model objects". While usually used interchangeably, the domain entities and model object can also be instances of active record pattern (basically: domain objects with added storage logic).

In ordinary domain object there is no storage logic. It gets handled by data mappers.

The term "model objects" comes from Fowler's books (read PoEAA for more details), and, IMHO, is part of the confusions MVC, because the entire model is an application layer (MVC consists of it and presentation layer), which contains those "model objects", which are usually dealt with by services (in that image, the model layer is all three concentric circles together).

I much prefer to use "domain object" term instead.

The term "domain entity" (or "entity object") is usually used when author implies that the object is a direct representation of a storage structure (more often - a database table). These are also almost always implementations of active record.

_{P.S.: in some articles you would also see term "models" (plural). It usually is not directly related to MVC design pattern, because it talks about Rails-like architecture, where "models" are just active records, that get directly exposed-to/created-by controller.}

.. not sure whether this confused you more

Answer 2

The terms are a bit vague I agree. I would use domain to refer to the business area you are dealing with. Like banking or insurance or what not. Then you have domain models. These are the things you deal with in that business domain, like for domain of banking you have accounts, customers, transfers etc. I would use term entity for referencing the class/POJO or the persisted / concrete version of a model.

What probably confuses you here is that in the term MVC, the model is a concrete thing, but it references the data model used to represent some presentation in a web GUI so don't get that mixed in with the above explanation.