As you probably already know, the specification template allows you to send a filter to your repository (among other cases). I have seen many implementations for this.
Usually, people provide another method in the specification interface that represents the expression tree that should be sent to the Entity Framework:
public interface ISpecification<T> { bool IsSpecifiedBy(T item); Expression<Func<T, bool>> GetPredicate() }
The GetPredicate repository GetPredicate method and passes it to the Where method in DbSet EF. In this way, you have limited what expressions will be generated, and you guarantee that it will generate a valid SQL statement.
To include logical operators in the specification, you need to mix the expressions together. There is this post from Vladimir Khorikov, where he explains in detail how to do this.
I usually don’t like this solution because it assumes that your domain model matches the persistence model . Most people are okay with that. But I like to keep things VERY separated onion architecture .
From my own experience, I found that eventually the Entity Framework will pollute your domain model with dbcontexts, EF attributes, open setters, properties that make sense only for the database, etc.
Therefore, I usually keep two separate models (classes), the essence of "constancy" is very simple and very similar to the database schema, and the essence of the "domain" is enriched by behavior and invariants.
And this creates a problem for the solution described above, since the specification is based on the domain model and cannot have dependencies for the constancy model.
Therefore, you will need to navigate through the composite specification and create it to create the predicate. Visitor is a good design template for this.
I recently wrote a series of posts where I explain