独特的约束技巧-实体框架

Question

Since there is not any way to add unique constraint to entities in current latest EF version I came up with an idea. ( I know constraints can be added by embedding SQL )

I added a method to my Repository class :

private IDbSet<T> Entities;

public virtual bool Contains(Func<T, bool> predicate)
{
        return Entities.Any(t => predicate(t));
}

I think this will replace my current duplicate prevention logic:

bool already exists = (from t in someSet.Entities 
                       where t.UniqueColumn == entity.UniqueColumn select t).
                       FirstOrDefault() != null;

if(alreadyExists)
throw new Exception("Entity already exists in DB.");

Is this a good idea, what is the best way to prevent duplicate insertions / updates to database?

EDIT : I edited the Contains method to use a predicate. Now I can write a code like this:

if(repository.Contains(t=> t.Email == "someemail@email.com")
     throw new Exception("Email already exists.");

Answer 1

If this is going to be a rare situation, then there's nothing wrong with placing a unique constraint on the database and then catching the exception that gets generated when you try to insert an existing record.

If this is likely to happen a lot, then I would still place the constraint on the database, but do the check as you are.

FWIW, you should make your method take an Expression<Func<T, bool>> so that the query does not get converted to an Enumerable.

Answer 2

Well, this won't work because you need to declare what you want to enforce uniqueness on. On the primary key? On some other column like "UserName"? On the User.EMail column, but only if it is not null (so you can have multiple users with a null EMail at the same time)?

I prefer such things to be explicit. So I'd actually prefer the logic that you had previously.

Answer 3

在数据库中实施唯一性的最佳方法是使用唯一性约束 。