LINQ To SQL奇怪的联接问题

Question

I have a simple database with two tables. Users and Configurations. A user has a foreign key to link it to a particular configuration.

I am having a strange problem where the following query always causes an inner join to the Configuration table regardless of the second parameter value. As far as I can tell, even though the "UserConfiguration =" part of the object initialisation is conditional, LINQ doesn't see that and determines that a relationship is followed in any case.

If I actually remove that last initialisation, the whole thing works as expected. It doesn't inner join when loadConfiguration == false and it does join when loadConfiguration == true.

Anyone got any ideas about this? Is this syntax just not going to work? The only thought I have now is to wrap the return in a basic if statement - I just wanted to avoid the duplicated lines.

public UserAccount GetByUsername(string username, bool loadConfiguration)
{
    using (Database database = new Database())
    {
        if (loadConfiguration)
        {
            DataLoadOptions loadOptions = new DataLoadOptions();
            loadOptions.LoadWith<User>(c => c.Configuration);
            database.LoadOptions = loadOptions;
        }

        return (from c in database.Users
                where c.Username == username
                select new UserAccount
                {
                    ID = c.ID,
                    ConfigurationID = c.ConfigurationID,
                    Username = c.Username,
                    Password = c.Password.ToArray(),
                    HashSalt = c.HashSalt,
                    FirstName = c.FirstName,
                    LastName = c.LastName,
                    EmailAddress = c.EmailAddress,

                    UserConfiguration = (loadConfiguration) ? new ApplicationConfiguration
                    {
                        ID = c.Configuration.ID,
                        MonthlyAccountPrice = c.Configuration.MonthlyAccountPrice,
                        TrialAccountDays = c.Configuration.TrialAccountDays,
                        VAT = c.Configuration.VAT,
                        DateCreated = c.Configuration.DateCreated

                    } : null

                }).Single();
    }
}

Thanks in advance,

Martin.

Answer 1

I dont think it will work like that.

I suggest splitting it into 2 distinct queries.

There are probably better ways, but it would require more 'plumbling'.

Answer 2

Nope, this will not work. I've run into similar issues many times. The reason for that has to do with expressions at compile time vs. conditions at run-time.

You could do 2 queries or if you don't mind joining to configuration regardless of loadConfiguration parameter, you could use:

var q = (from c in database.Users
                where c.Username == username
                select c).Single();

and then use Linq-to-Objects on the result.

Answer 3

Replace .Single() with SingleOrDefault() and Linq will switch to a left outer join. I don't know if in your case it will do it, but in some cases it does.

Edit dint's see the Single was for the entire query and not for the configuration part:

try this:

     UserConfiguration = (loadConfiguration && c.Configuration != null) ? new ApplicationConfiguration
     {
          ID = c.Configuration.ID,
          MonthlyAccountPrice = c.Configuration.MonthlyAccountPrice,
          TrialAccountDays = c.Configuration.TrialAccountDays,
          VAT = c.Configuration.VAT,
          DateCreated = c.Configuration.DateCreated
      } : null