With the old driver I could specify the fields I wanted to return from a query as follows:
var cursor = Collection.Find(query).
SetFields(Fields<MealPlan>.Exclude (plan => plan.Meals));
How do I accomplish this with the 2.0 driver?
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2 Answers
You need to use the Projection method on IFindFluent (which is what Find and Projection return):
var findFluent = Collection.Find(query).Projection(Fields<MealPlan>.Exclude (plan => plan.Meals))
Now, this would eventually generate a cursor of BsonDocuments since it doesn't know how the projection looks. You can call the generic Projection instead to add that type:
var findFluent = Collection.Find(query).Projection<MealPlan>(Fields<MealPlan>.Exclude (plan => plan.Meals))
In a more general sense (which is less relevant when using Exclude), you could also specify fields using a lambda expression:
var findFluent = Collection.Find(query).Projection(plan => plan.Meals)
8 Comments
Graeme
Thanks. I know the driver is in beta, but will the generic projection support abstract classes? I'm assuming the serializer has access to the discriminator to determine the concrete type.
Craig Wilson
What exactly do you mean?
i3arnon
@Graeme Do you mean something like
Collection.Find(query).Projection<AbstractPlan>(Fields.Include("_id"))? No, it won't work as it tries to create an instance out of AbstractPlan. It can work if the base class isn't abstract but the instance would be of the base class and not the concrete type.Craig Wilson
Actually, I think that as long as the discriminator was there (in the projection), it should work. It would certainly come back typed as the base class and require a downcast, but underneath, in this case, we are just using the AbstractPlan serializer and it knows how to deal with discriminators. I haven't tried this though, so...
i3arnon
@CraigWilson just did. It does work when the discriminator is added (which unlike
_id isn't returned by default from mongo) |
If you want SetFields back you can write your own extension method:
public static class MongoExtensions
{
public static IFindFluent<T, T> SetFields<T>(this IFindFluent<T, T> query, params string[] fields)
{
if ( fields == null || fields.Length == 0 )
{
return query;
}
var project = Builders<T>.Projection.IncludeAll<T>(fields);
return query.Project<T>(project);
}
public static ProjectionDefinition<T> IncludeAll<T>(this ProjectionDefinitionBuilder<T> projection,
params string[] fields)
{
ProjectionDefinition<T> project = null;
foreach (string columnName in fields)
{
if (project == null)
{
project = Builders<T>.Projection.Include(columnName);
}
else
{
project = project.Include(columnName);
}
}
return project;
}
}
