How to read back automatically generated identifier from mongo after pasting using official C # driver?

Question

after you enter a new document in mongodb through the official C # driver, how do you immediately read the generated _id so that I can use it as a “foreign” key for other collections? I know in sql server, I can immediately read the value of the identifier column for the newly inserted row, so I need similar functionality in mongodb.

since the _id generated by mongo is not an actual member of the object, suppose you need to do something with a generic bsondocument?

Answer 1

You can do upsert with the findAndModify command to achieve the same effect with less work than creating your own identifiers. (Why worry, there is a very good reason 10gen decided to use sceme, which makes scalding easy)

The findAndModify command allows you to find or update (create if it does not exist) a document and return the same document.

The general view is as follows:

db.runCommand( { findAndModify : <collection>, <options> } ) 

Learn more about this here .

You would like to use the new addition to the upsert option to return a newly created object.

Answer 2

In MongoDB, identifiers are (usually) generated on the client side. And you can create it yourself, using the appropriate driver call, insert it into the document, and it will be saved.

I did not work with the C # driver, but the Ruby driver does all the work for me.

 ruby-1.9.3-p0 :027 > obj = coll.insert({'foo' => 'bar'}) => BSON::ObjectId('4ef15e7f0ed4c00272000001') ruby-1.9.3-p0 :030 > coll.find.to_a => [{"_id"=>BSON::ObjectId('4ef15e7f0ed4c00272000001'), "foo"=>"bar"}] 

Here is how I can create a new ID

 ruby-1.9.3-p0 :039 > newid = BSON::ObjectId.new => BSON::ObjectId('4ef15f030ed4c00272000002') ruby-1.9.3-p0 :040 > coll.insert({_id: newid, test: 'test'}) => BSON::ObjectId('4ef15f030ed4c00272000002') ruby-1.9.3-p0 :041 > coll.find.to_a => [{"_id"=>BSON::ObjectId('4ef15e7f0ed4c00272000001'), "foo"=>"bar"}, {"_id"=>BSON::ObjectId('4ef15f030ed4c00272000002'), "test"=>"test"}] 

Answer 3

If you need _id, you can create it yourself and set it manually in the document.

Answer 4

In most drivers, the _id field _id actually created on the client side before moving to the server. MongoDB does not use the identifier "auto-increment", so you can generate a random identifier and tell the server to "use this".

In C #, the code is as follows:

 var id = ObjectId.GenerateNewId(); 

So you can create a BSON document and just save it:

 var toSave = new BsonDocument { { "_id", ObjectId.GenerateNewId() }, { "data", "my data" } }; db.collection.Save(toSave); 

However, by default, when you .Save() in a document, this will update the _id field. That way, you can just save the BSONDocument (or BSONSerializable ) class and then read it back.

Note that there is a DBRef specification that simplifies the implementation of "foreign keys." The docs here in C # you will want to look at the DBRef class.

Answer 5

Like the other answers here, identifiers are assigned on the client side. Something you can do is create a default convention that generates a new identifier during insertion, if it is not already set.

 public class DefaultValueConvention : MongoDB.Bson.Serialization.Conventions.IDefaultValueConvention { public object GetDefaultValue(MemberInfo memberInfo) { var type = memberInfo.MemberType == MemberTypes.Property ? ((PropertyInfo) memberInfo).PropertyType : ((FieldInfo) memberInfo).FieldType; if (type.IsSubclassOf(typeof(ObjectId))) return ObjectId.GenerateNewId(); else return null; } } 

And configure the driver to use this convention:

 var profile = new ConventionProfile(); profile.SetDefaultValueConvention(new DefaultValueConvention()); BsonClassMap.RegisterConventions(profile, x => x.FullName.StartsWith("ConsoleApplication")); 

So now you can create an object and save it in two lines:

 var animal = new Animal {Name = "Monkey", PercentDeviationFromHumans = 2.01}; db["animals"].Save(animal); 

Actually, with the latest driver, you don’t even need to set a standard default agreement, it already has this OOTB behavior. Despite this, agreements in mangoes are not used enough.