How do I get the latest document inserted in (a standalone, no RS) MongoDb over existing collections?
And how do I get all documents inserted after this document?
It can be done only in replica set. Please follow the tutorial to convert standalone instance to replica set.
You can get a reference to the last inserted document from oplog:
db.oplog.rs.find({op:"i"}).sort({$natural: -1}).limit(1);
ns field contains name of the database and collection, and o._id contains the object's identifier.
To get references to documents that were inserted after that later you can use ts field of the document you retrieved in the previous query:
db.oplog.rs.find({op:"i", ts: {$gt: last.ts}});
This command will cause MongoDB to load everything to memory, if oplog.rs is very big, then it will cause memory usage high
db.oplog.rs.find({op:"i"}).sort({$natural: -1}).limit(1);
Related
I have a mongodb collection called employeeInformation, in which I have two documents:
{"name1":"tutorial1"}, {"name2":"tutorial2"}
When I do db.employeeInformation.find(), I get both these documents displayed. My question is - is there a query that I can run to confirm that the collection contains only those two specified documents? I tried db.employeeInformation.find({"name1":"tutorial1"}, {"name2":"tutorial2"}) but I only got the id corresponding to the first object with key "name1". I know it's easy to do here with 2 documents just by seeing the results of .find(), but I want to ensure that in a situation where I insert multiple (100's) of documents into the collection, I have a way of verifying that the collection contains all and only those 100 documents (note I will always have the objects themselves as text). Ideally this query should work in mongoatlas console/interface as well.
db.collection.count()
will give you number of inserts once you have inserted the document.
Thanks,
Neha
I would like to find out how old/stale a collection is, I was wondering if there was a way to know when the last query was made to a collection, or even get a list of all collections last access date.
If your Mongodb collection document _id is of the following format "_id" : ObjectId("57bee0cbc9735bf0b80c23e0") then Mongodb stores the create document timestamp.
This can be retrieved by executing the following query
db.newcollection.findOne({"_id" : ObjectId("57bee0cbc9735bf0b80c23e0")})._id.getTimestamp();
the result would be an ISODate like this ISODate("2016-08-25T12:12:59Z")
find out how old/stale a collection
There is no predefined libraries available in mongodb to track the oldness of a collection. But it is doable by maintaining a log where we can keep an entry when we are accessing a collection.
References
ObjectID.getTimestamp()
Log messages
Rotate Log files
db.collection.stats()
I want to delete all the collections from my db which are not used for long time. Is there any why i can check when the particular collection was last used?
It depends what you mean by 'last used'. If you mean the last time a document was inserted into the collection then you could do this by converting the ObjectId of the last inserted document into a date. The following query should return the date the last document was inserted:
db.<collection_name>.findOne({},{_id:1})._id.getTimestamp()
the findOne query will return documents in natural order, therefore if you input no query criteria ('{}') then it will return the most recently inserted document. You can then get the _id field and call the getTimestamp() function
I'm not sure if there is any way to reliably tell when a collection was last queried. If you're running your database with profiling enabled then there might be entries in the db.system.profile collection, or in the oplog.
I have some unused collections in the MongoDb database. I've to find out when the CRUD operations done against collections in the database. We have our own _id field instead of mongo's default object_id. We dont have any time filed in the collections to find out the modification time. is there any way to find out the modification time of collections in mongodb from meta data? Is there any data dictionay informations like in oracle to find out this? please give some idea/workarounds
To make a long story short: MongoDB has a flexible schema. Simply add a date field. Since older entries don't have it, they can not be the last entry.
Let's call that field mtime.
So after adding a date field to your schema definition, we generate an index in descending order on the new field:
db.yourCollction.createIndex({mtime:-1})
Finding the last mtime for a collection now is easy:
db.yourCollection.find({"mtime":{"$exists":true}}).sort({"mtime":-1}).limit(1)
Do this for every collection. When the above query does not return a value within the timeframe you defined for purging a collection, simply drop it, since it has not been modified since you introduced the mtime field.
After your collections are cleaned up, you may remove the mtime field from your schema definition. To remove it from the documents, you can run a simple query:
db.yourCollection.update(
{ "mtime":{ $exists:true} },
{ "$unset":{ "mtime":""} },
{ multi: true}
)
There is no "data dictionary" to get this information in MongoDB.
If you've enabled the profiling level in advance to log all operations (db.setProfilingLevel(2)) and you haven't had many operations to log, so that the system.profile capped collection hasn't overwritten whatever logs you are interested in, you can get the information you need there—but otherwise it's gone.
We are planning on using MongoDB _id as a key that we would provide to the client. Therefore, the requirement is that this key should not change if we ever need to move the data from one collection to another. The copy will be performed using db.copyDatabase() or mongoimport.
One of the ways in which data can be copied from one collection to another is iterating through the documents in the first collection(C1) and inserting these documents in the second collection(C2). In this case _id should remain the same(in C2) because it would be present in the documents(of C1) being inserted(same as the case in which we would provide an _id ourselves).
However, if there is an alternate way in which documents are copied, the _id might change since it depends on :
(1) The UNIX timestamp
(2) Machine identifier
(3) ProcessId
(**This should only happen if MongoDB while copying removes _id from documents in C1 and regenerated them while inserting into C2?)
We want the _id values to be same irrespective of the location of the destination collection:
(1)within same database
(2)different database - same machine
(3)different database - different machine)
Thanks
No, the _id numbers will not change.
A new ObjectId is generated when a document without an _id field is inserted into the database. When you insert a document which already has an _id field, MongoDB won't touch it.
The timestamp, machine identifier and processID refer to those where the ObjectID was generated. This can be a database server, but it can also be generated by the MongoDB driver on the application server. In that case MongoDB will not change it on its own.
By the way: The _id can be an auto-generated ObjectId, but it doesn't have to. You can also use any other value as _id, as long as you can guarantee that it's unique. So when your data already has a natural key, you can use this as _id when you want to.