具有2个连接的实体框架在同一表上连接第1列或第2列

Question

I am trying to construct an EF query to either return column 1 or column 2.

Column 1 is pulled from a different table based on the relationships/join of the query.

Column 2 is pulled from the joined table.

This is for a survey/answer model. Some answers are hardcoded (eg user picks from a list) and some are free text.

Free text get saved in a table, and the user picked value is stored in a different table.

End goal is to have a POCO class with the question and answer:

public class SurveyResponse
{
    public string Question { get; set; }
    public string Answer { get; set; }
}

My query looks like:

var surveyResponses = (from ias in dataContext.AnswerSet
                       join answer in dataContext.Answer on ias.AnswerSetID equals answer.AnswerSetID
                       join oa in dataContext.OptionalAnswer on answer.PossibleAnswerID equals oa.OptionalAnswerID // this line loses valuetext answer
                       where ias.LeadID.Equals(dbLead.leadID)
                       select new SurveyResponse() { Question = "question(todo)", Answer = (answer.ValueText != "" ? answer.ValueText : oa.Title)}).ToList();

And here is some of the data which shows why the join on optionalanswer loses the last row

PossibleAnswerID    ValueText
979D3ADF-4F9C-4F4D-AC9C-034CE960003A    
EF488701-683D-4855-A9A7-7F9D78468E63    
D442F616-6D4B-4176-9F08-133E2C655BE4    
5FD57A95-B385-4CAE-ADAB-1711CEA00A08    
NULL    Questions about sales

Answer 1

Apparently your query generates inner join in sql and thus it eliminates Answer with empty PossibleAnswerID . So you just need to change it to left join . It can be done like this

var surveyResponses = 
    (from ias in dataContext.AnswerSet
     join answer in dataContext.Answer on ias.AnswerSetID equals answer.AnswerSetID
     join oa in dataContext.OptionalAnswer on answer.PossibleAnswerID equals oa.OptionalAnswerID into optans // this line loses valuetext answer
     from oa in optans.DefaultIfEmpty()
     where ias.LeadID.Equals(dbLead.leadID)
     select new SurveyResponse() { Question = "question(todo)", Answer = (answer.ValueText != "" ? answer.ValueText : oa.Title)}).ToList();

Answer 2

If you use fluent syntax, .Select can accept an Expression<Func<TEntity, TResult>> argument.

Simplified:

Expression<Func<AnswerSetType, SomeClass>> func = SomeMethod();

dataContext.AnswerSet
    .Select(func)
    .ToList();

SomeClass could maybe simply be this depending on what exactly you need:

public class SomeClass 
{
    public string ValueFromColumn1OrColumn2 { get; set; }
}

SomeMethod is where you would define which value needs to be returned from your query:

public Expression<Func<AnswerSetType, SomeClass>> SomeMethod()
{
    if (condition)
    {
        return (answerSetType => new SomeClass { ValueFromColumn1OrColumn2 = answerSetType.Column1 });
    }
    else
    {
        return (answerSetType => new SomeClass { ValueFromColumn1OrColumn2 = answerSetType.Column2 });
    }
}

In the Expression you can also make use of navigation properties to access columns in a different table.

I am making the assumption here that both your columns have the same datatype.