When working with NoSQL<\/i> databases, your documents might be quite heavy and in some cases, you would like to get only a slice of original data. For instance, let’s assume we have an Account<\/i> document which among other things contains a list of transactions<\/p>\n As there might be hundreds of transaction in the account object, you might want to occasionally work on a subset of original data, say for instance AccountSlim<\/i> object to improve the performance<\/p>\n MongoDB.Driver<\/i> has a couple of ways of defining projections so as you can operate on a slimmer object instead of a default one. For instance<\/p>\n Unfortunately, those are a client-side projection. This means that the entire object is returned from the database and we just serialized it to a different class. You can check it on your own by examining request and result commands sent to Mongo<\/i><\/p>\n In both cases requested query doesn’t contain “projection” section, so entire document is returned <\/p>\n Of course, there is a possibility to manually create a server-side projection, for instance<\/p>\n or with a strongly typed version<\/p>\n However, in my opinion, this is error-prone and it would be better to generate server-side projection automatically based on properties in a slim object. As I didn’t find anything like that in the official driver, here is my approach for handling this<\/p>\n In order to create a custom projection, all we have to do is to extend ProjectionDefinition<TSource, TResult><\/i> class and provide a RenderedProjectionDefinition<\/i> with all properties which are in both “heavy” and “slim” object<\/p>\n As you can see we use MongoDB.Driver<\/i> build-in projections to render our custom projection consisting of necessary properties. Note, that as we are using StringFieldDefinition<\/i> instead of defining Bson<\/i> document manually, the projection will take into account potential class mappings or attribute mappings applied to your object <\/p>\n Having the projection ready we can make it a bit easier to use by introducing some extensions. The first one looks as follows<\/p>\n which allows you to use this projection similarly like others \u2013 so by accessing Builders<\/i> class<\/p>\n The second method will extend IFindFluent<TDocument, TProjection><\/i> interface<\/p>\n and thanks to it we will end up with an even better syntax<\/p>\n One way or another we will end up with proper projection definition which will result in a smaller document returned from the database<\/p>\n\r\npublic class Account : IAccountDefinition\r\n{\r\n public ObjectId Id { get; set; }\r\n\r\n public string Name { get; set; }\r\n\r\n public List\r\npublic class AccountSlim : IAccountDefinition\r\n{\r\n public ObjectId Id { get; set; }\r\n \r\n public string Name { get; set; }\r\n }\r\n<\/pre>\n2.Exploring existing options <\/h3>\n
\r\nList
\r\nIAsyncCursor
\r\nvar clientSettings = MongoClientSettings.FromUrl(new MongoUrl(\"mongodb:\/\/localhost:27017\")); \r\nclientSettings.ClusterConfigurator += builder => \r\n{ \r\n builder.Subscribe\r\n\/\/ query \r\n{ \r\n\r\n \"find\" : \"Accounts\", \r\n\r\n \"filter\" : { \r\n\r\n \"_id\" : ObjectId(\"5dac6ab1ea342a254e1b7748\") \r\n\r\n }, \r\n\r\n \"$db\" : \"Accounting\", \r\n\r\n \"lsid\" : { \r\n\r\n \"id\" : CSUUID(\"c6b40361-c47f-4fe6-99bd-6a9ec1702576\") \r\n\r\n } \r\n\r\n} \r\n\r\n\/\/ returned document \r\n\r\n{ \r\n\r\n \"cursor\" : { \r\n\r\n \"firstBatch\" : [{ \r\n\r\n \"_id\" : ObjectId(\"5dac6ab1ea342a254e1b7748\"), \r\n\r\n \"AccountName\" : \"WRKYDWKKJY\", \r\n\r\n \"Transactions\" : [{ \r\n\r\n \"_id\" : ObjectId(\"5dac6ab1ea342a254e1b7749\"), \r\n\r\n \"Amount\" : \"413648280\", \r\n\r\n \"CreatedAt\" : ISODate(\"2019-10-20T14:09:53.635Z\"), \r\n\r\n \"ModifiedAt\" : ISODate(\"2019-10-20T14:09:53.635Z\") \r\n\r\n }] \r\n\r\n }], \r\n\r\n \"id\" : NumberLong(0), \r\n\r\n \"ns\" : \"Accounting.Accounts\" \r\n\r\n }, \r\n\r\n \"ok\" : 1.0 \r\n\r\n} \r\n<\/pre>\n\r\nvar accountSlims = await accountsCollection.Find(defaultAccountFilterDefinition)\r\n .Project(new ObjectProjectionDefinition
\r\nvar bsonDocument = Builders
3. Class-based server-side projection<\/h3>\n
\r\nusing System.Collections.Generic;\r\nusing System.Collections.ObjectModel;\r\nusing System.Linq;\r\nusing System.Reflection;\r\nusing MongoDB.Bson.Serialization;\r\nusing MongoDB.Driver;\r\n\r\nnamespace MongoDBServerSideProjection.Projections\r\n{\r\n public class ServerSideProjectionDefinition\r\nBsonClassMap.RegisterClassMap
\r\nusing MongoDB.Driver;\r\nusing MongoDBServerSideProjection.Projections;\r\n\r\nnamespace MongoDBServerSideProjection.Extensions\r\n{\r\n public static class ProjectionDefinitionBuilderExtensions\r\n {\r\n public static ServerSideProjectionDefinition\r\nvar projection = Builders
\r\nusing MongoDB.Driver;\r\n\r\nnamespace MongoDBServerSideProjection.Extensions\r\n{\r\n public static class FindFluentExtensions\r\n {\r\n public static IFindFluent\r\nList
\r\n\/\/ query\r\n{ \r\n\r\n \"find\" : \"Accounts\", \r\n\r\n \"filter\" : { \r\n\r\n \"_id\" : ObjectId(\"5dac846eea29403cb83f99bb\") \r\n\r\n }, \r\n\r\n \"projection\" : { \r\n\r\n \"_id\" : 1, \r\n\r\n \"AccountName\" : 1 \r\n\r\n }, \r\n\r\n \"$db\" : \"Accounting\", \r\n\r\n \"lsid\" : { \r\n\r\n \"id\" : CSUUID(\"cf5d5a88-2598-4110-9295-6417d1a5f7dd\") \r\n\r\n } \r\n\r\n} \r\n\r\n\/\/ returned document\r\n\r\n{ \r\n\r\n \"cursor\" : { \r\n\r\n \"firstBatch\" : [{ \r\n\r\n \"_id\" : ObjectId(\"5dac846eea29403cb83f99bb\"), \r\n\r\n \"AccountName\" : \"CBBRRWQVKJ\" \r\n\r\n }], \r\n\r\n \"id\" : NumberLong(0), \r\n\r\n \"ns\" : \"Accounting.Accounts\" \r\n\r\n }, \r\n\r\n \"ok\" : 1.0 \r\n}\r\n<\/pre>\n