The $slice operator is used to return a subset of an array. The Enumerable.Take method can be used to create a $slice operation on a array field.
Method call
Description
Take(1)
Returns the first element
Take(N)
Returns the first N elements
Take(-1)
Returns the last element
Take(-N)
Returns the last N elements
Get first N elements
The sample returns Traveler documents but only include the first element of their VisitedCountries array field.
Slice.cs
var travelersQueryableCollection = tripsDatabase
.GetCollection<Traveler>(Constants.TravelersCollection)
.AsQueryable();
var sliceQuery = from t in travelersQueryableCollection
select new {
t.Name, visitedCountries =
t.VisitedCountries.Take(1) // slice here
};
var sliceQueryResults = await sliceQuery.ToListAsync();
public class Traveler
{
[BsonId]
public ObjectId Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
public List<string> Activities { get; set; }
public List<VisitedCountry> VisitedCountries { get; set; }
}
The same result can be achieved using a ProjectionDefinition.
var sliceProjection = Builders<Traveler>.Projection
.Expression(u =>
new
{
name = u.Name,
visitedCountries = u.VisitedCountries.Take(1)
});
var sliceProjectionResults = await travelersCollection
.Find(Builders<Traveler>.Filter.Empty)
.Project(sliceProjection)
.ToListAsync();
Get last N elements
The following sample returns the Traveler documents but only the last 2 visited countries included.
Slice.cs
var sliceQueryTwoLastCountries =
from t in travelersQueryableCollection
select new
{
t.Name,
visitedCountries = t.VisitedCountries.Take(-2)
};
var sliceQueryTwoLastCountriesResults =
await sliceQueryTwoLastCountries
.ToListAsync();
public class Traveler
{
[BsonId]
public ObjectId Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
public List<string> Activities { get; set; }
public List<VisitedCountry> VisitedCountries { get; set; }
}
Pagination
Slice can be combined with the Skip method and provide full pagination functionality. The following sample skips the first 2 VisitedCountries array elements and returns the next 3.
var travelersQueryableCollection = tripsDatabase
.GetCollection<Traveler>(Constants.TravelersCollection)
.AsQueryable();
var sliceWithSkipQuery =
from t in travelersQueryableCollection
select new {
t.Name,
visitedCountries = t.VisitedCountries
.Skip(2).Take(3) };
var sliceWithSkipQueryResults =
await sliceQuery.ToListAsync();
public class Traveler
{
[BsonId]
public ObjectId Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
public List<string> Activities { get; set; }
public List<VisitedCountry> VisitedCountries { get; set; }
}
Filter operator - $filter
The $filter operator is used to match and return array elements that fulfill the specified condition. The Enumerable.Where method can be used to create the condition.
The sample returns Traveler documents with their VisitedCountries array field containing only the countries that have been visited once.
ComparisonOperators.cs
var travelersQueryableCollection = tripsDatabase
.GetCollection<Traveler>(Constants.TravelersCollection)
.AsQueryable();
var filterQuery =
from t in travelersQueryableCollection
select new
{
t.Name,
visitedCountries = t.VisitedCountries
.Where(c => c.TimesVisited == 1)
};
var filterQueryResults = await filterQuery.ToListAsync();
public class Traveler
{
[BsonId]
public ObjectId Id { get; set; }
public string Name { get; set; }
public int Age { get; set; }
public List<string> Activities { get; set; }
public List<VisitedCountry> VisitedCountries { get; set; }
}
Multiply operator - $multiply
The $multiply operator is used to multiply numbers and return the result. The operator can be used with both raw and field values.
Assuming a collection contains Order documents in the following format...
The sample creates a projection stage to return the total for each order, with total=price∗quantity
Multiply.cs
var ordersCollection = tripsDatabase
.GetCollection<Order>(Constants.OrdersCollection);
var multiplyQuery =
from o in ordersCollection.AsQueryable()
select new // creates a projection stage
{
o.OrderId,
total = o.Quantity * o.Price // $multiply field
};
var multiplyQueryResults =
await multiplyQuery.ToListAsync();
public class Order
{
[BsonId]
public int OrderId { get; set; }
public string Item { get; set; }
public int Price { get; set; }
public int Quantity { get; set; }
[BsonIgnoreIfDefault]
public int? LotNumber { get; set; }
public ShipmentDetails ShipmentDetails { get; set; }
}
