How to get "value" from a field received in a "lambda expression"

Estou fazendo um ORM e no meu 'where' and encontrei um problema específico ao utilizar um atributo de um objeto na condição

public zDatabase zWhere<TSource>(Expression<Func<TSource, bool>> expression)
        {
            string header = expression.Parameters[0].Name;

            MemberExpression memberExpr = (MemberExpression)expression.Body;

            var WhereString = expression.Body.ToString();
            Where = PrepairWhere(WhereString, header);

            return this;
        }

But if I use like

databaseInstance.zWhere<Midia>(x => x.type == "zip")

The parameter is received like

x => (x.type == "zip")

This is the expected result, but when using as an attribute as the example below ...

databaseInstance.zWhere<Products_Price>(x => x.product_id== product_id)

I get the following code

x => (x.produto_id == value(AERendererFastFood.Model.DAO.Products_Products_PriceDAO+<>c__DisplayClass0_0).product_id)

And, indeed, I'd like to get the value of the expression attribute