How do I write an MVC Entity Framework select query from the controller using passed string arguments?

Question

I created a search tool in MVC. The user types the searchTerm string into a search box and selects options for the search from 2 dropdown select lists. From the first list the user selects "Fuzzy" or "Literal" for the searchType, and from the second list the user can select a specific field to search, specField. The 3 string values are passed to the controller once the search button is clicked.
I am trying to write an "if" statement that queries the Entity Framework DB model based on these 3 string values passed into the controller. How do I write queries that use these values? This is what I have. I know the queries aren't written correctly:

 Public PartialViewResult _Search(string searchTerm, string specField, string searchType)
 {
      //This works to populate the table with the entire DB 
      _db = new IntegrationDBEntities();
      var request = (from r in _db.Requests
                     select r).ToList();

     //These two are not correct and I need help writing these
     if (searchType == "Fuzzy" && searchTerm != "" && specField != "None")
     {
          request = (from r in _db.Requests
                          where r.(specField).Contains(searchTerm)
                          select r).ToList();
     }
     if (searchType == "Literal" && searchTerm != "" && specField == "None")
     {
          request = (from r in _db.Requests
                         where r.Equals(searchTerm)
                         select r).ToList();
     }
     ViewData.Model = request;
     Return partialView();
 }  

Answer 1

I am not 100% sure about how you Request object looks like, but I would use PredicateBuilder . Here is an example of what your search function could look like.

IQueryable<Request> SearchRequests (string searchTerm, string specField, string searchType)
{
    var predicate = PredicateBuilder.False<Request>();

    //These two are not correct and I need help writing these
    if (searchType == "Fuzzy" && searchTerm != "" && specField != "None")
    {
        predicate = predicate.Or (r => r.(specField).Contains(searchTerm));
    }
    if (searchType == "Literal" && searchTerm != "" && specField == "None")
    {
        predicate = predicate.Or (r => r.Equals(searchTerm));
    }

    return new IntegrationDBEntities()
                  .Requests
                  .AsExpandable()
                  .Where(predicate);
}

Then your method could look like this:

Public PartialViewResult _Search(string searchTerm, string specField, string searchType)
 {
     ViewData.Model = SearchRequests(searchTerm, specField, searchType).ToList();
     Return partialView();
 } 

UPDATE : I added working example from LinqPad. You will need to modify it slightly to get it to work for you with EntityFramework .

void Main()
{
    var results = SearchRequests("Fuzzy", "later").ToList();
    results.Dump();

    var results2 = SearchRequests("Literal", "Test me now").ToList();
    results2.Dump();
}

// Define other methods and classes here
public class Request
{
    public int Id {get;set;}
    public string SearchTerm {get;set;}
}

public IQueryable<Request> LoadData()
{
    var list = new List<Request>();
    list.Add(new Request {Id = 1, SearchTerm = "Test me now"});
    list.Add(new Request {Id = 2, SearchTerm = "Test me later"});
    list.Add(new Request {Id = 3, SearchTerm = "Test me maybe"});
    list.Add(new Request {Id = 4, SearchTerm = "Test me now"});
    list.Add(new Request {Id = 5, SearchTerm = "Test me later or never"});
    list.Add(new Request {Id = 6, SearchTerm = "Test me maybe or today"});

    return list.AsQueryable();
}

public IQueryable<Request> SearchRequests (string searchType, string searchTerm)
{
    var data = LoadData();
    var predicate = PredicateBuilder.False<Request>();
    //These two are not correct and I need help writing these
    if (searchType == "Fuzzy")
    {
        predicate = predicate.Or(r => r.SearchTerm.Contains(searchTerm));
    }
    if (searchType == "Literal")
    {
        predicate = predicate.Or (r => r.SearchTerm.Equals(searchTerm));
    }

    return data.Where(predicate);
}

public static class PredicateBuilder
{
    public static Expression<Func<T, bool>> True<T> ()  { return f => true;  }
    public static Expression<Func<T, bool>> False<T> () { return f => false; }

    public static Expression<Func<T, bool>> Or<T> (this Expression<Func<T, bool>> expr1,
                                                        Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
        return Expression.Lambda<Func<T, bool>>
            (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
                                                        Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
        return Expression.Lambda<Func<T, bool>>
            (Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters);
    }
}

Results for first search:

Id  SearchTerm
2   Test me later
5   Test me later or never

Results for second search:

Id  SearchTerm
1   Test me now
4   Test me now

Answer 2

Hope this works with no errors:

ParameterExpression pe = Expression.Parameter(typeof(Request), "Request");
Expression property = Expression.Property(pe, specField);
Expression predicate = null;
Expression srch = Expression.Constant(searchTerm, typeof(string));
if(searchType == "Fuzzy")
{
    MethodInfo contains = typeof(string).GetMethod("Contains", new Type[] { typeof(string) });
    predicate = Expression.Call(property, contains, srch);
}
else
{
    predicate = Expression.Equal(property, srch);
}

MethodCallExpression where = Expression.Call(
    typeof(Queryable),
    "Where",
    new Type[] { _db.Requests.ElementType },
    _db.Requests.Expression,
    Expression.Lambda<Func<Request, bool>>(predicate, new ParameterExpression[] { pe }));

IQueryable<Request> result = _db.Requests.Provider.CreateQuery<Request>(where);

var list = result.ToList();

Let me know if you have any trouble