I have the following situation: There are Movies, which have a title (and more properties) There are Users, which have a name (and more properties) Users can “own” zero to many movies Every user can rate every movie (even the ones they don’t own), and their rating is remembered on a per-user basis …

In this case as owned and rated movies are indepent things and they related to a user, User type itself could have fields: owned : Set MovieId, rated : Dict MovieId Rating. Also try not use use primitive types (String, Int) for representing domain data, use custom types. But the actual structure s…

So, Here is how you build it. There are Movies , which have a title (and more properties) type alias MovieId = Int type alias Movie = { id : MovieId, title : String, plot : String, } There are Users , which have a name (and more properties) type alias User = { id : Int, fi…

Thank you both for your suggestions: Since you both used ID based solutions: Is there a way to ensure consistency? For example, how to make sure that a user doesn’t “own” a movieId which doesn’t exist?

If thats the case then the Movies type should have fields called owned : List UserId, ratedBy : List UserId not the User type. This will stay consistent as long as User is not removed or deleted.

I don’t think there’s any good way of modelling away your “impossible state” without introducing a tighter (and probably undesired) coupling between Movie and User. You are dealing with two distinct entities. Consistency between them isn’t guaranteed at a type level. I’d ask myself whether the stat…

Where is this persisted? If in a relational DB like Postgres, you are better off using referential integrity in the DB.

In this approach you don’t enforce it in model because maintaining proper relations would take quite some code and unless what you are doing is a MySQL competitor just not worth it. If user rated movie while it was available then keep it and filter out all deleted ones in rendering.

Thank you @alexander-js , I really like this solution! You mentioned: [image] alexander-js: without introducing a tighter (and probably undesired) coupling between Movie and User . Just out of curiosity, could you sketch what this would look like? Having a hard time imagining it. @Sebastian …

Adding to what @alexander-js said: Isn’t the set of movies that exist really the set of movies that someone owns? If I make a vacation movie on my phone, is that a legit movie that some people (my hobby filmmakers’ club) are allowed to rate? It depends on your use case but I think it should be allow…

How to 'make illegal state irrepresentable' in this situation?

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opsb March 7, 2022, 9:20pm 10

This thread and the linked talk by Richard Feldman provide some good thoughts on this topic Model as a relational database for components

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How to 'make illegal state irrepresentable' in this situation?

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