This may be a really stupid question, but I do not have it clear in my mind as to how it is best to manage this so want to put it down here and see what is common practice.
Coming from .net my web applications are never 1 project which is everything, I tend to at least have a data layer project which deals with persisting entities to the database and making these models represent said entity in a DB friendly manner. Then I have my UI project which has its own models which are a representation of the entity for the UI, which may have validation based information and will most likely be a more cut down model only exposing what is needed.
So the main point I am trying to get out here is that although we may have a User entity, the models to represent that may be slightly different in the UI and Data layers...
Now fast forwarding to rails, you create your project and it comes with database connectivity (which I believe can be swapped out to any flavour you want), and validation and a whole manner of other frameworks and functionality. This then seems to imply that I no longer need 2 projects, just 1 as its all included within here and all done for me, so all I need to worry about are making my models.
This is the part where I am a little confused, as lets say I am using ActiveRecord I will need to make my model and inherit from ActiveRecord::Base, then I will need to setup how this model connects to other models etc, so I have my model's data concerns sorted, now I need to setup my UI concerns, about validation and string lengths etc... now where do these go... I am assuming on the same model, but then a model isnt just a simple representation of data, its a blob of stuff containing concerns for databases and views and who knows what else.
It just seems a little odd to put everything within this one place. I know in .net there are plenty of examples where with large object graph representations in the DB the data models are VERY different to UI models, so is it wise to couple them into one model this way, or is Ruby and its frameworks so flexible in this area that you do not have these same problems? If so is there some example or article which can solidify in my mind how you sidestep the normal problems that cause you to separate your concerns to have maintainable code...
=== Edit ===
Just to try and clear up any confusion, in my post when I say my view models and view concerns, I do not mean presentation concerns. I am sorry if it came across that way. What I mean is that I may have (in a normal .net example) a UserStorage model, which has concerns about persisting a User entity. I then in the ui layer have a view which displays many users and one that displays single users in more detail. You may have 2 differing models here a UserSummary model and a UserDetails model, both partially represent a User entity, but are customised for the actual view in question, as you may get to a situation where UserDetails also becomes a composition of a User and a Company entity, which would mean there are 2 storage based classes feeding into 1 view based class.
Within the examples and guides it makes out like you should have 1 view model which deals with these concerns, as well as storage concerns, and in this case it just seems like if I were in the situation where I had a view model that was a composition of a User and Company it would seem odd for this view layer class to worry about its storage, as its never stored as itself, it is stored as 2 separate models in the database/datastore.
This is the REAL problem I am trying to get at here, it seems to be tightly coupling your view to your storage concerns which I am used to being 2 different things which should never be mixed... like putting your main course and pudding on the same plate...
In vanilla Rails, there is no such thing as a "view model".
The models handle all business logic, including query and persistence, association, and validation. You seem to be dismissing validation as a concern of the view, but this is actually a core concern for the integrity of your models: it does belong there.
That's not to say that validation shouldn't happen in the view (client-side) too, but the model has your core business rules and is the place where the validation rules are ultimately checked.
You will want your models to hold most of the logic of your application. This means things like checking if a user is valid or active, finding related records, etc. Pretty much anything that isn't presentational belongs in the model.
In the view, Rails provides helper methods for formatting. These come from modules that are included in the view instance. Helper methods have access to the instance variables of the view (your models) and are used to present them.
In some situations, passing naked models into the view makes it hard to keep things modular and organized. Some people prefer using Presenters to wrap the models before passing them to the view. It can help to organize your code, but it's also an extra layer. Presenters are not standard in Rails, but it's trivial to add this pattern using plain ruby objects or a library like draper if it makes sense for your application.
EDIT: Oh look, a most excellent blog post on just this very topic came up today, from the most excellent Thoughtbot.