To search for a document in MongoDB you use the Find method on a IMongoCollection<T> reference. Find method accepts a FilterDefinition<T> parameter where T is the collection's type.
Filters can be created using the Builders<T>.Filter definition builder which contain multiple filters. The following example finds a user document based on its Id. It does this using the equality filterEq<T> on the id field.
ReadDocuments.cs
// Get the collectionvar personsCollection =usersDatabase.GetCollection<User>("users");// Create an Equality Filter Definitionvar personFilter =Builders<User>.Filter .Eq(person =>person.Id,appPerson.Id);// Find the document in the collection var personFindResult =await personsCollection .Find(personFilter).FirstOrDefaultAsync();
// Get the collectionvar bsonPersonCollection =usersDatabase.GetCollection<BsonDocument>("users");// Create a bson filtervar bsonPersonFilter =Builders<BsonDocument>.Filter.Eq("_id",appPerson.Id);// Find a person using a class filtervar bsonPersonFindResult =await bsonPersonCollection .Find(bsonPersonFilter).FirstOrDefaultAsync();// alternative for passing a filter argumentbsonPersonFindResult =await bsonPersonCollection.Find(newBsonDocument("_id",appPerson.Id)).FirstOrDefaultAsync();
// You need to know the Id's value and typedb.users.findOne({"_id":ObjectId("5e5d11fe152a428290f30245")})
Notice that when filter used with BsonDocument, _id field name used instead of Id which is the property name on the User class. When you use Builders<User>.Filter this is done automatically for you by the driver, based on the Id serialization settings
Find multiple documents
To search for multiple documents follow the same process but this time use the ToList method. The following example finds all documents with female gender.
ReadDocuments.cs
// Get the collectionvar personsCollection =usersDatabase.GetCollection<User>("users");// Create an equality filter on a user's gendervar femaleGenderFilter =Builders<User>.Filter .Eq(person =>person.Gender,Gender.Female);// Find all documents having female gendervar females =await personsCollection .Find(femaleGenderFilter).ToListAsync();
// Get the collectionvar bsonPersonCollection =usersDatabase.GetCollection<BsonDocument>("users");// Create an equality filter on gendervar bsonFemaleGenderFilter = Builders<BsonDocument> .Filter.Eq("gender",Gender.Female);// Get all female usersvar bsonFemales =await bsonPersonCollection .Find(bsonFemaleGenderFilter).ToListAsync();
