Question

Using MySQl 5.5.19, entity framework 4.3, and have tried both the Sun-provided MySQL connector and the DevArt one without any difference between the two.

So, I’ve got two statements which produce different queries. The first is:

var desserts = context.desserts
    .AsQueryable()
    .OrderBy(m => m.Id)
    .Where(m => m.FlavorId == 123)
    .ToList();

and the second is

var desserts = context.desserts
    .AsQueryable()
    .OrderBy(m => m.Id)
    .Where(m => m.FlavorId == someFlavorId)
    .ToList();

The first query produces a SQL query that looks something like this:

SELECT
  Extent1.Id,
  Extent1.Name,
  Extent1.FlavorId
FROM icecream.dessert AS Extent1
WHERE Extent1.FlavorId = 123
ORDER BY Extent1.Id ASC

The second one produces a much more costly query that looks something like this:

SELECT
  Project1.Id,
  Project1.Name,
  Project1.FlavorId
  FROM(
    SELECT
      Extent1.Id,
      Extent1.Name,
      Extent1.FlavorId
    FROM icecream.dessert AS Extent1
    WHERE Extent1.FlavorId = 123
  ) AS Project1
  ORDER BY Project1.Id ASC

And, of course, the latter query is the one that is used in my production code. I’m confused why these two result in a different query.

Any ideas how to fix this?

Answer 1

They produce different sql, because the first query you are passing in a constant and the second one you are passing in a parameter.

If you want to mimic the first query you would need to create an Expression for the Where predicate like this:

ParameterExpression parameter = Expression.Parameter(typeof(Dessert), "d");
Expression propertyOrField = Expression.PropertyOrField(parameter , "FlavorId");
Expression constant = Expression.Constant(someFlavorId, typeof(long?));
Expression equal = Expression.Equal(propertyOrField, constant);

var lambda = Expression.Lambda<Func<Dessert, bool>>(equal, parameter);

var desserts = context.desserts
    .AsQueryable()
    .OrderBy(m => m.Id)
    .Where(lambda)
    .ToList();