mORMot Open Source

bigheart

Member

Registered: 2014-08-01

Posts: 53

How to get the last inserted document in mongoDB with mORMot

Hi!

I'm using mOROot for my private project.

And i want to know how to fetch the latest document from mongoDB server with Direct Access synMongoDB

In googling, i can find some command for mongoDB

db.foo.find().sort({$natural:1}).limit(1);

How to implement above command to FindDoc or FindJSON like below:

Client := TMongoClient.Create('host');
DB := Client.Database['dbname'];

Coll := DB.CollectionOrCreate['collect'];
Coll.FindDoc('{ $sort: {_id : -1 } } ', []);

help me please.

ab

Administrator

From: France

Registered: 2010-06-21

Posts: 14,620

Website

Re: How to get the last inserted document in mongoDB with mORMot

You should use TMongoCollection.AggregateDoc() (or AggregateJSON).

/// calculate aggregate values using the MongoDB aggregation framework
    // and return the result as a TDocVariant instance
    // - the Aggregation Framework was designed to be more efficient than the
    // alternative map-reduce pattern, and is available since MongoDB 2.2 -
    // see http://docs.mongodb.org/manual/referenc … /aggregate
    // - you should specify the aggregation pipeline as a list of JSON object
    // operators (without the [..]) - for reference of all available phases,
    // see http://docs.mongodb.org/manual/core/agg … n-pipeline
    // - if the server sent back no {result:...} member, will return null
    // - if the server sent back one item as {result:[{..}]}, will return
    // this single item as a TDocVariant
    // - if the server sent back several items as {result:[{..},{..}]}, will
    // return a dvArray kind of TDocVariant
    // - for instance, the following will return the maximum _id value of
    // the collection:
    // ! AggregateDoc('{$group:{_id:null,max:{$max:"$_id"}}}',[]).max
    function AggregateDoc(Operators: PUTF8Char; const Params: array of const): variant;

Then you specify the $sort argument, possibly:

AggregateDoc('{ $sort : { $natural : 1 } }, { $limit : 1 }',[])

See http://docs.mongodb.org/manual/referenc … tion/sort/

bigheart

Member

Registered: 2014-08-01

Posts: 53

Re: How to get the last inserted document in mongoDB with mORMot

Thanks for your advice.

But when i execute as below:

Doc := Coll.AggregateDoc('{ $sort : { $natural : 1 } }, { $limit : 1 }',[]);

Null is returned to Doc variable.

Of course, Doc := Coll.FindDoc(); is OK!

What is the problem for getting the document?

Give me more advice, please.

Thanks in advance.

bigheart

Member

Registered: 2014-08-01

Posts: 53

Re: How to get the last inserted document in mongoDB with mORMot

I have solved above problem as:

Doc := Coll.AggregateDoc('{ $sort : { _id: 1 } }, { $limit : 1 }',[]);

not $natural but _id.

Thanks.

ab

Administrator

From: France

Registered: 2010-06-21

Posts: 14,620

Website

Re: How to get the last inserted document in mongoDB with mORMot

Weird.
http://docs.mongodb.org/manual/referenc … ta/natural states that it should work.

But, by design, _id are generated in increasing order...
As a consequence, your query also does the trick.

BTW why are you searching for the latest document ID?
Usually, you have it before adding the document, on client side, since the _id is generated before sending the document to the server.
Either you explicitly set the _id field before calling Insert or Save, or you have the CreatedObjectID: PBSONObjectID optional parameter in our TMongoCollection.Insert() and TMongoCollection.Save() methods to get the generated ID.

bigheart

Member

Registered: 2014-08-01

Posts: 53

Re: How to get the last inserted document in mongoDB with mORMot

I do make some device's monitoring program.

The device status data is saved using TMongoCollection.Insert method by client #1 program to MongoDB in every second.

And the client #2 query the saved data which last inserted document and display them to monitor.

I'm not just want a document ID but a last inserted data.