我个人不会使用$facet
在这里,因为你只有一条管道,这违背了目的$facet
首先...
以下更简单并且可扩展性更好($facet
将创建一个可能庞大的文档)。
db.collection.aggregate([
{
$match: {
"Name" : "My First Book"
}
}, {
$project: {
"Tags": {
$objectToArray: "$Tags"
}
}
}, {
$unwind: "$Tags"
}, {
$sortByCount: "$Tags"
}, {
$group: { // not really needed unless you need to have all results in one single document
"_id": null,
"categorizedByTags": {
$push: "$$ROOT"
}
}
}, {
$project: { // not really needed, either: remove _id field
"_id": 0
}
}])
可以使用 C# 驱动程序编写如下:
var collection = new MongoClient().GetDatabase("test").GetCollection<Book>("test");
var pipeline = collection.Aggregate()
.Match(b => b.Name == "My First Book")
.Project("{Tags: { $objectToArray: \"$Tags\" }}")
.Unwind("Tags")
.SortByCount<BsonDocument>("$Tags");
var output = pipeline.ToList().ToJson(new JsonWriterSettings {Indent = true});
Console.WriteLine(output);
这是使用侧面的版本:
var collection = new MongoClient().GetDatabase("test").GetCollection<Book>("test");
var project = PipelineStageDefinitionBuilder.Project<Book, BsonDocument>("{Tags: { $objectToArray: \"$Tags\" }}");
var unwind = PipelineStageDefinitionBuilder.Unwind<BsonDocument, BsonDocument>("Tags");
var sortByCount = PipelineStageDefinitionBuilder.SortByCount<BsonDocument, BsonDocument>("$Tags");
var pipeline = PipelineDefinition<Book, AggregateSortByCountResult<BsonDocument>>.Create(new IPipelineStageDefinition[] { project, unwind, sortByCount });
// string based alternative version
//var pipeline = PipelineDefinition<Book, BsonDocument>.Create(
// "{ $project :{ Tags: { $objectToArray: \"$Tags\" } } }",
// "{ $unwind : \"$Tags\" }",
// "{ $sortByCount : \"$Tags\" }");
var facetPipeline = AggregateFacet.Create("categorizedByTags", pipeline);
var aggregation = collection.Aggregate().Match(b => b.Name == "My First Book").Facet(facetPipeline);
var output = aggregation.Single().Facets.ToJson(new JsonWriterSettings { Indent = true });
Console.WriteLine(output);