Is in Mongo any way to do check and setting like atomic operation ? I am making booking for hotels and if there is free room you can reserve, but what if two or more people want to reserve in same time. Is there anything similar to transaction in Mongo or any way to solve this problem ?
Yes, that's the classic use case for MongoDB's findAndModify command.
Specifically for pymongo: find_and_modify.
All updates are atomic operations over a document. Now find_and_modify locks that document and returns it back in the same operation.
This allows you to combine a lock over the document during find and then applies the update operation.
You can find more about atomic operations:
http://www.mongodb.org/display/DOCS/Atomic+Operations
Best,
Norberto
The answers reference findAndModify documentation. But a practical example given the OP's requirements will do justice:
const current = new ISODate();
const timeAgoBy30Minutes = new Date(current.getTime() - 1000 * 30 ).toISOString();
db.runCommand(
{
findAndModify: "rooms",
query: {
"availability" : true,
"lastChecked" : {
"$lt": timeAgoBy30Minutes
}
},
update: { $set: { availability: false, lastChecked: current.toISOString() } }
}
)
In the above example, my decision to use db.runCommand verses db.rooms.findAndModify was strategic. db.runCommand will return a status code as to whether the document was updated, which allows me to perform additional work if the return value was true. findAndModify simply returns the old document, unless the new flag is passed to the argument list by which it will return the updated document.
Related
as the title says, I want to perform a find (one) for a document, by _id, and if doesn't exist, have it created, then whether it was found or was created, have it returned in the callback.
I don't want to update it if it exists, as I've read findAndModify does. I have seen many other questions on Stackoverflow regarding this but again, don't wish to update anything.
I am unsure if by creating (of not existing), THAT is actually the update everyone is talking about, it's all so confuzzling :(
Beginning with MongoDB 2.4, it's no longer necessary to rely on a unique index (or any other workaround) for atomic findOrCreate like operations.
This is thanks to the $setOnInsert operator new to 2.4, which allows you to specify updates which should only happen when inserting documents.
This, combined with the upsert option, means you can use findAndModify to achieve an atomic findOrCreate-like operation.
db.collection.findAndModify({
query: { _id: "some potentially existing id" },
update: {
$setOnInsert: { foo: "bar" }
},
new: true, // return new doc if one is upserted
upsert: true // insert the document if it does not exist
})
As $setOnInsert only affects documents being inserted, if an existing document is found, no modification will occur. If no document exists, it will upsert one with the specified _id, then perform the insert only set. In both cases, the document is returned.
Driver Versions > 2
Using the latest driver (> version 2), you'll use findOneAndUpdate as findAndModify was deprecated. The new method takes 3 arguments, the filter, the update object (which contains your default properties, that should be inserted for a new object), and options where you have to specify the upsert operation.
Using the promise syntax, it looks like this:
const result = await collection.findOneAndUpdate(
{ _id: new ObjectId(id) },
{
$setOnInsert: { foo: "bar" },
},
{
returnOriginal: false,
upsert: true,
}
);
const newOrUpdatedDocument = result.value;
Its a bit dirty, but you can just insert it.
Be sure that the key has a unique index on it (if you use the _id it's ok, it's already unique).
In this way if the element is already present it will return an exception that you can catch.
If it isn't present, the new document will be inserted.
Updated: a detailed explanation of this technique on the MongoDB Documentation
Here's what I did (Ruby MongoDB driver):
$db[:tags].update_one({:tag => 'flat'}, {'$set' => {:tag => 'earth' }}, { :upsert => true })}
It will update it if it exists, and insert it if it doesn't.
Is is possible to do a MongoDB increment ($inc) and return the result of the increment as part of the same call, rather than having to make two calls?
You will have to use findandModify().
From the docs:
Modifies and returns a single document. By default, the returned document does not include the modifications made on the update. To return the document with the modifications made on the update, use the new option. The findAndModify() method is a shell helper around the findAndModify command.
You can use as such:
db.yourCollection.findAndModify({
query:{},
update: { $inc: { propertyToIncrement: 1 }},
new: true
})
I have a mongoDB document that has the following structure:
{
user:user_name,
streams:[
{user:user_a, name:name_a},
{user:user_b, name:name_b},
{user:user_c, name:name_c}
]
}
I want to use $pullAll to remove from the streams array, passing it an array of streams (the size of the array varies from 1 to N):
var streamsA = [{user:"user_a", name:"name_a"},{user:"user_b", name:"name_b"}]
var streamsB = [{name:"name_a", user:"user_a"},{name:"name_b", user:"user_b"}]
I use the following mongoDB command to perform the update operation:
db.streams.update({name:"user_name", {"$pullAll:{streams:streamsA}})
db.streams.update({name:"user_name", {"$pullAll:{streams:streamsB}})
Removing streamsA succeeds, whereas removing streamsB fails. After digging through the mongoDB manuals, I saw that the order of fields in streamsA and streamsB records has to match the order of fields in the database. For streamsB the order does not match, that's why it was not removed.
I can reorder the streams to the database document order prior to performing an update operation, but is there an easier and cleaner way to do this? Is there some flag that can be set to update and/or pullAll to ignore the order?
Thank You,
Gary
The $pullAll operator is really a "special case" that was mostly intended for single "scalar" array elements and not for sub-documents in the way you are using it.
Instead use $pull which will inspect each element and use an $or condition for the document lists:
db.streams.update(
{ "user": "user_name" },
{ "$pull": { "streams": { "$or": streamsB } }}
)
That way it does not matter which order the fields are in or indeed look for an "exact match" as the current $pullAll operation is actually doing.
I would like to speed up an query on my mongoDB which uses $where to compare two fields in the document, which seems to be really slow.
My query look like this:
db.mycollection.find({ $where : "this.lastCheckDate < this.modificationDate})
What I would like to do is add a field to my document, i.e. isCheckDateLowerThenModDate, on which I could execute a probably much faster query:
db.mycollection.find({"isCheckDateLowerThenModDate":true})
I quite new to mongoDB an have no idea how to do this. I would appreciate if someone could give me some hints or examples on
How to initialize such a field on an existing collection
How to maintain this field. Which means how to update this field when lastCheckDate or modificationDate changes.
Thanks in advance for your help!
You are thinking in a right way!
1.How to initialize such a field on an existing collection.
Most simple way is to load each document (from your language), calculate this field, update and save.
Or you could perform an update via mongo shell:
db.mycollection.find().forEach(function(doc) {
if(doc.lastCheckDate < doc.modificationDate)
{
doc.isCheckDateLowerThenModDate = true;
}
else
{
doc.isCheckDateLowerThenModDate = false;
}
db.mycollection.save(doc);
});
2.How to maintain this field. Which means how to update this field when
lastCheckDate or modificationDate changes.
You have to do it yourself from your client code. Make some wrapper for update, save operations and recalculate this value each time there. To be absolutely sure that this update works -- write unit tests.
The $where clause is slow because it is evaluating each document using the JavaScript interpreter.
There are a few alternatives:
1) Assuming your use case is "look for records that need updating", take advantage of a sparse index:
add a boolean field like needsChecking and $set this whenever the modificationDate is updated
in your "check" procedure, find the documents that have this field set (should be fast due to the sparse index)
db.mycollection.find({'needsChecking':true});
after you've done whatever check is needed, $unset the needsChecking field.
2) A new (and faster) feature in MongoDB 2.2 is the Aggregation Framework.
Here is an example of adding a "isUpdated" field based on the date comparison, and then filtering the matching documents:
db.mycollection.aggregate(
{ $project: {
_id: 1,
name: 1,
type: 1,
modificationDate: 1,
lastCheckDate: 1,
isUpdated: { $gt:["$modificationDate","$lastCheckDate"] }
}},
{ $match : {
isUpdated : true,
}}
)
Some current caveats of using the Aggregation Framework are:
you have to specify fields to include aside from _id
the result is limited to the current maximum BSON document size (16Mb in MongoDB 2.2)
Considering a simple mongo document structure:
{ _id, firstTime, lastTime }
The client needs to insert a document with a known ID, or update an existing document. The 'lastTime' should always be set to some latest time. For the 'firstTime', if a document is being inserted, then the 'firstTime' should be set to current time. However, if the document is already created, then 'firstTime' remain unchanged. I would like to do it purely with upserts (to avoid look ups).
I've crawled the http://www.mongodb.org/display/DOCS/Updating, but I just don't see how that particular operation can be done.
I don't believe this is something unreasonable, there are $push and $addToSet operations that effectively do that on array fields, just nothing that would do the same on simple fields. It's like there should be something like $setIf operation.
I ran into the exact same problem and there was no simple solution for <2.4 however since 2.4 the $setOnInsert operator let's you do exactly that.
db.collection.update( <query>,
{ $setOnInsert: { "firstTime": <TIMESTAMP> } },
{ upsert: true }
)
See the 2.4 release notes of setOnInsert for more info.
I ran into a very similar problem when attempting to upsert documents based on existing content--maybe this solution will work for you also:
Try removing the _id attribute from your record and only use it in the query portion of your update (you'll have to translate from pymongo speak...)
myid = doc.get('_id')
del doc['_id']
mycollection.update({'_id':myid}, {'$set':doc}, upsert=True)
If you will trigger the following code 2 subsequent times, it will first set both firstVisit and lastVisit on document insert (and will return upsertedId in the response) and on the second it will only update lastVisit (and will return modifiedCount: 1).
Tested with Mongo 4.0.5 though I believe should be working with older versions.
db.collection.updateOne(
{_id: 1},
{
$set: {
lastVisit: Date.now()
},
$setOnInsert: {
firstVisit: Date.now()
}
},
{ upsert: true }
);
There's no way to do this with just one upsert. You'd have to do it as 2 operations - first try to insert the document, if it already exists the insert will fail due to duplicate key violation on the _id index. Then you do an update operation to set the lastTime to now.