Question

I’m building a dynamic query that can have n of Where method calls and n of SelectMany calls dependent upon user input. For example I may have:

var qZ = entityContext.TableA
        .SelectMany(a=>a.TableB, (a,t)=>new{a,t}  )
        .Where(a=>a.t.FieldID==21)
        .Where(a=> EntityFunctions.Left(a.t.Value,1)=="p")
        .SelectMany(a=>a.a.TableC, (a,t)=>new{a,t}  )
        .Where(a=>a.t.FieldID==22)
        .Where(a=> a.a.t.Value=="Peter" && a.t.Value=="Pan")
        .Where(a=> a.a.a.TypeID==3)
        .Select(a=> new{ a.a.a.ItemID }
        ).Distinct();

In the method I’m writing, I use helper methods that return an IQueryable as seen in the return line below.

return query.Provider.CreateQuery(
    Expression.Call(typeof(Queryable), 
      "Where", 
       new Type[] {query.ElementType}, 
       query.Expression, predicateLambda)
           );

I’m able to create LambdaExpressions for all of the various query attribute-value pairs required, but I am unable to create one for the resultSelector of Queryable.SelectMany.

How can we create (a,t) => new{a=a, t=t} in an expression tree? Or How do we accomplish the same result as the .SelectMany above using Expression.Call like below?

Expression.Call(typeof(Queryable), 
      "SelectMany", 
       ????????, 
       ????????
           );

I’ve tried using the SelectMany overload that doesn’t require the resultSelector which works to some degree, however, I don’t know how to reference the properties of t in subsequent method calls.

I’ve found this lambda expression ((a,t) => new{a=a, t=t}) associated with SelectMany all over the web, but I can’t find any example of how to convert it to an expression tree.

UPDATE:
Let’s reframe the question. I can pass the lambda like this

var q = entityContext.TableA.AsQueryable();

var q1 = Queryable.SelectMany(q, a => a.TableB, (a, t) => new { a = a, t = t });

var q2 = Queryable.Where(q1,a=>a.t.FieldID==22);

That works, however, since I don’t know ahead of time how many SelectMany need to be called and since each call changes to anonymous type of the IQueriable, is there a way to cast (and re-cast) the anonymous type to a single variable? This way I can loop through and apply whatever method necessary to the variable and then enumerate to get the results once the query is built. Something like:

var q = entityContext.TableA..AsQueryable();

q = Queryable.SelectMany(q, a => a.TableB, (a, t) => new { a = a, t = t });

q = Queryable.Where(q,a=>a.t.FieldID==22);

(BTW: This doesn’t work)

Answer 1

The way that I ended up resolving this required a paradigm shift. The first query above was based upon the fact that I learned to write queries by joining all the tables I needed together to give me access to filter on and select fields in those tables.

SelectMany() creates joins and the box around my thinking at the time required that if I need to filter on a specific column in a table, I had to join that table to my query. This in turn changed the type of my IQueryable resulting in my not being able to predict the Type of the IQueryable at design time.

Answer:

Step 1: Set the type of IQueryable to the output type it needs to return. In the case above, the result was always IQueryable.

Step 2: Utilize Expressions to dynamically create the WHERE predicate, including any and all tables necessary to create the proper filter. This always returns Expression> and all of the other variables we easily accounted for. And rememeber, in EF it isn’t necessary to join table outside of Where() if they are only needed in Where().