Build dynamic Expression for search - silverlight

Question

I'm in trouble, can't figure out seems a very simple thing that in plain SQL can be done within 1 minute, it's been several hours so far. Here is the situation:
I have single field where user may enter as many words as he/she likes
I need to build Expression to find match
Let's say there are 3 fields in database: firstname, middlename, lastname
I need to split the search entry and compare against those 3 fields
I'm dealing with Silverlight RIA, EF
Once again search entry contains UNKNOWN number of words
Here under what I'm trying to accomplish, return type is mandatory:
public Expression<Func<MyEntity, bool>> GetSearchExpression(string text)
{
Expression<Func<MyEntity, bool>> result;
var keywords = text.Trim().Split(" ");
foreach(var keyword in keywords)
{
// TODO:
// check whether 'OR' is required (i.e. after second loop)
// (firstname = 'keyword'
// AND
// middlename = 'keyword'
// AND
// lastname = 'keyword')
// OR
// (firstname like '%keyword%'
// AND
// middlename like '%keyword%'
// AND
// lastname like '%keyword%')
}
return result;
}
Thanks in advance!

Answer 1

The simplest thing would be to use Joe Albahari's PredicateBuilder to do something like this:
var predicate = PredicateBuilder.False<MyEntity>();
foreach (string keyword in keywords)
{
string temp = keyword;
predicate = predicate.Or (
p => p.FirstName.Contains (temp) &&
p.LastName.Contains (temp) &&
p.MiddleName.Contains (temp));
}
return predicate;
I left out the equality-checks because "Contains" (i.e. like '%...%') will cover that possibility anyway.
I do have to point out, though, that your conditions don't make any sense, from a business logic standpoint. Under what circumstances do you want to find someone whose first, last, and middle name all contain "John"? I suspect what you really want is something more like this:
var predicate = PredicateBuilder.True<MyEntity>();
foreach (string keyword in keywords)
{
string temp = keyword;
predicate = predicate.And (
p => p.FirstName.Contains (temp) ||
p.LastName.Contains (temp) ||
p.MiddleName.Contains (temp));
}
return predicate;
One final note: Because PredicateBuilder requires you to call .AsExpandable() when you are creating your query, I don't know whether this will work for you. You might have to resort to building your own expressions, which can be somewhat tedious. This can get you started, though:
var pParam = Expression.Parameter(typeof(MyEntity), "p");
var predicate = Expression.Constant(true);
foreach (string keyword in keywords)
{
var keywordExpr = Expression.Constant(keyword);
// TODO: create an expression to invoke .FirstName getter
// TODO: create an expression to invoke string.Contains() method
//TODO: do the same for lastname and middlename
predicate = Expression.And(predicate,
Expression.Or(
Expression.Or(firstNameContainsKeyword,
middleNameContainsKeyword),
lastNameContainsKeyword));
}
return Expression.Lambda<Func<MyEntity, bool>>(predicate, pParam);