Entity Framework中按字符串的MIN和MAX值

Question

I have database table. The ORM for it is:

public long Id { get; set; }
public System.Guid AttrId { get; set; }
public System.Guid ProdId { get; set; }
public string Value { get; set; }

public virtual Attributes Attributes { get; set; }
public virtual Products Products { get; set; }

As you can see value is a string type. I'm trying to get min and max values by this field (some values represented as double). So here is my method for this:

public double[] GetMaxMinVals(Guid attrId)
{
    double[] res = new double[2];
    using(entityContext = new SiteDBEntities())
    {
        res[0] = entityContext.ProductAttributes.Where(x=>x.AttrId == attrId)
            .Min(x => Convert.ToDouble(x.Value));
        res[1] = entityContext.ProductAttributes.Where(x => x.AttrId == attrId)
            .Max(x => Convert.ToDouble(x.Value));
    }

    return res;
}

But I getting exception:

LINQ to Entities does not recognize the method 'Double ToDouble(System.String)' method, and this method cannot be translated into a store expression.

So how can I search for a string value like a decimal?

Answer 1

The problem here is your query will be translated into SQL and run on the database, and Entity Framework doesn't know how to translate Convert.ToDouble into valid SQL code.

So you could cast to double as below, which will be later converted to SQL CAST AS statement.

res[0] = entityContext.ProductAttributes.Where(x=>x.AttrId == attrId).Min(x => (double)x.Value);
res[1] = entityContext.ProductAttributes.Where(x => x.AttrId == attrId).Max(x => (double)x.Value);

Answer 2

First you can cast the Value to double then use Max/Min :

public double[] GetMaxMinVals(Guid attrId)
    {
        double[] res = new double[2];
        using(entityContext = new SiteDBEntities())
        {
            res[0] = entityContext.ProductAttributes
              .Where(x => x.AttrId == attrId)
              .Select(x => x.Value)
              .Cast<double>()
              .Min();
        }

        return res;
    }

Answer 3

In EF Dbcontext don't support "Convert.ToDouble", you can fix same that:

public double[] GetMaxMinVals(Guid attrId)
        {
            double[] res = new double[2];
            using(entityContext = new SiteDBEntities())
            {
                res[0] = entityContext.ProductAttributes.Where(x=>x.AttrId == attrId).Min(x => (double)x.Value);
                res[1] = entityContext.ProductAttributes.Where(x => x.AttrId == attrId).Max(x => (double)x.Value);
            }

            return res;
        }