Historical value of entity

Question

Historical value of entity

LoGA80 opened this issue 10 days ago · 6 comments

I have a generic repository solution that relies on DbSets and - among others - I use it to query an item based on the primary key. For this I can use the convenient Find method that accepts an object array with the key values. I started to use temporal tables because there has been a need to present the state of entities for a given time but as soon as I add any temporal query (e.g.: TemporalAsOf) I no more have a DbSet but an IQueryable and I loose the option to use Find to get a specific entity, all examples I was able to find was using a Where expression with a specific filter condition).

Currently I have a hacky solution by using the code from the Github source of EF core but it has to use a method that is marked as internal so not a future-proof solution.

var entityParameter = Expression.Parameter(typeof(TEntity), "e"); var expression = Expression.Lambda<Func<TEntity, bool>>(ExpressionExtensions.BuildPredicate(dbContext.Entry<TEntity>(new()).Metadata.FindPrimaryKey().Properties, new ValueBuffer(id), entityParameter), entityParameter); var historicalEntity = dbContext.Set<TEntity>().TemporalAsOf(timestamp).AsTracking().FirstOrDefault(expression);

I need a reliable generic way to be able to query a historical version of an entity either by having a Find that I can apply after TemporalAsOf or a way to request a historical value of an already located TEntity object (so first Find by key and then look for history). I would only use this for querying so I don't need all the capabilities of DbSet, only the Find by primary key.

Answer 1 · 2024-12-11T17:21:41.000Z

@LoGA80 Find() really is important mostly if you may already be tracking an entity but aren't sure (i.e. you may loaded it before): it first looks in EF's change tracker and only performs the query against the database if the entity wasn't found. In all other cases, it's just as good to simply use a Where() clause and specify the key properties there. That's what I'd recommend doing in this case.

Answer 2 · 2024-12-11T17:33:48.000Z

My problem with Where is that I cannot keep this piece of code generic any more, I would need to implement dedicated GetById methods for the entities. I could imagine something like an IQueryable<TEntity>.ByKey(object[] key) method that could use the same expression as internally used by Find (when not found in tracker).

Answer 3 · 2024-12-11T18:02:47.000Z

I'm not sure I'd recommend being "generic" here with your GetById; in fact, Find() has the disadvantage of not being strongly-typed for what it is you're retrieving (an array of objects is generally not a great fit for a statically typed language like C#). I'd recommend having dedicated GetById() repository methods for each entity type - just like you need other dedicated methods for type-specific searches.

Answer 4 · 2024-12-11T18:02:52.000Z

@LoGA80 If all your entity types have the same kind of key, then you can do something like this with a non-generic IQueryable:

    var x = await queryable.Cast<object>()
        .SingleAsync(e => EF.Property<int>(e, "Id") == id);