Entity Framework 4.0 Code-First Dynamic Query

问题

I would like to query a table based on a list of KeyValuePair. With a Model-First approach, I could do the following:

var context = new DataContext();
var whereClause = new StringBuilder();
var objectParameters = new List<ObjectParameter>();

foreach(KeyValuePair<string, object> pair in queryParameters)
{
    if (whereClause.Length > 0)
        whereClause.Append(" AND ");
    whereClause.Append(string.Format("it.[{0}] = @{0}", pair.Key));
    parameters.Add(new ObjectParameter(pair.Key, pair.Value));
}

var result = context.Nodes.Where(whereClause.ToString(), parameters.ToArray());

Now I'm using a Code-First approach and this Where method is not available anymore. Fortunately, I saw an article somewhere (I can't remember anymore) which suggested that I could convert the DbContext to a IObjectContextAdapter then call CreateQuery like this:

var result = ((IObjectContextAdapter)context)
                .ObjectContext.CreateQuery<Node>(whereClause.ToString(), parameters.ToArray());

Unfortunately, this throws an error:

'{ColumnName}' could not be resolved in the current scope or context. Make sure that all referenced variables are in scope, that required schemas are loaded, and that namespaces are referenced correctly.

Where {ColumnName} is the column specified in the whereClause.

Any ideas how I can dynamically query a DbSet given a list of key/value pairs? All help will be greatly appreciated.

回答1:

I think your very first problem is that in the first example you are using Where on the entity set but in the second example you are using CreateQuery so you must pass full ESQL query and not only where clause! Try something like:

...
.CreateQuery<Node>("SELECT VALUE it FROM ContextName.Nodes AS it WHERE " + yourWhere) 

The most problematic is full entity set name in FROM part. I think it is defined as name of the context class and name of the DbSet exposed on the context. Another way to do it is creating ObjectSet:

...
.ObjectContext.CreateObjectSet<Node>().Where(yourWhere)

来源：https://stackoverflow.com/questions/6297217/entity-framework-4-0-code-first-dynamic-query