First of, I'm quite new to mongodb. Here's my question I've not been able to find a solution to.
Let's say I have 3 different collections.
mongos> show collections
collectionA
collectionB
collectionC
I want to create a script that iterates over all collections ind this database and find the last inserted timestamp in each of these collections. Here's what works inside mongos.
var last_element = db.collectionA.find().sort({_id:-1}).limit(1);
printjson(last_element.next()._id.getTimestamp());
ISODate("2014-08-28T06:45:47Z")
1. Problem (Iterate over all collections)
Is there any possibility to to sth. like.
var my_collections = show collections;
my_collections.forEach(function(current_collection){
print(current_collection);
});
Problem here, the assignment for my_collections
does not work.
I get SyntaxError: Unexpected identifier
. Do I need to quote the 'show' statement ? Is it even possible ?
2. Problem (storing collection in js var)
I can workaround Problem 1 by doing this:
var my_collections = ["collectionA", "collectionB", "collectionC"];
my_collections.forEach(function(current_collection){
var last_element = db.current_collection.find().sort({_id:-1}).limit(1);
print(current_collection);
printjson(last_element.next()._id.getTimestamp());
});
The last_element.next()
produces the following error:
error hasNext: false at src/mongo/shell/query.js:124
It seems that last_element isn't saved correctly.
Any suggestions on what I'm doing wrong??
UPDATE
Neils answer lead me to this solution. In addition to his code I had to check if the function getTimestamp
really exist. For some 'virtual' collections there seem to be no _id property.
db.getCollectionNames().forEach(function(collname) {
var last_element = db[collname].find().sort({_id:-1}).limit(1);
if(last_element.hasNext()){
var next = last_element.next();
if(next._id !== undefined && typeof next._id.getTimestamp == 'function'){
printjson(collname + " >> "+next._id.getTimestamp());
}else{
print(collname + " undefined!! (getTimestamp N/A)")
}
}
});
There is the db.getCollectionNames()
helper method that does this for you. You can then implement your code:
db.getCollectionNames().forEach(function(collname) {
// find the last item in a collection
var last_element = db[collname].find().sort({_id:-1}).limit(1);
// check that it's not empty
if (last_element.hasNext()) {
// print its timestamp
printjson(last_element.next()._id.getTimestamp());
}
})
You probably also want a .hasNext()
check in there to cater for possible empty collections.