Entity Framework Core - 调用一个 Where linq 方法没有关系

Question

I'm a newcomer to EF core, and I have a problem when I try to do the following. I have some business logic that filters coffees based on the price:

if(request.MoreThen != null)
            {
                query = query.Where(c =>
                    c.Prices
                    .OrderByDescending(p => p.CreatedAt)
                    .First()
                    .Price
                    >= request.MoreThen);
            }

And it works all and well. However, when I try to extract this functionality into a private method, I get that the Prices relation is null.

Now, I have several places I need to referance the latest price, here's the second if:

            if(request.MoreThen != null)
            {
                query = query.Where(c =>
                    c.Prices
                    .OrderByDescending(p => p.CreatedAt)
                    .First()
                    .Price
                    >= request.MoreThen);
            }

I find this not DRY at all, so I wanted to extract the fetching of the price into a private method like so:

private decimal GetPrice(Coffee model) 
{
  return model.Prices
     .OrderByDescending(p => p.CreatedAt)
     .First()
     .Price
}

However, now when I substitute the query:

   query = query.Where(c => GetPrice(c) <= request.LessThen)

I get a weird error, and in the debugger every field is present except for the Included ones. ( Do note that I called the .Include ) method.

The full code sample is included in my open source Project on Github

EDIT: I'm using Postgres as the Database engine, and code first approach

So my question is, why are the .Prices null even though I've included them at the start of the method?

Answer 1

I couldn't figure out how to operate on the Coffee class itself, however you can create an extension method on IQueryable<Coffee> .

For example:

public static class Extensions
{
    public static IQueryable<Coffee> GetPriceMoreThen(this IQueryable<Coffee> query, decimal moreThen)
    {
        return query.Where(c =>
                           c.Prices
                            .OrderByDescending(p => p.CreatedAt)
                            .First()
                            .Price >= moreThen);
    }
}

which then you can call as such

query = query.GetPriceMoreThen(request.MoreThen);

Disclaimer: I'm making some assumptions on your model here