Are lokijs id's ($loki) unique?
if I have 2 documents from one collection with $loki:1 and $loki:2 as id's, and I delete $loki:1, the next one I create should it be $loki:3 ???
That's correct. Collections keep track of the last ID that was created and assign the a lastId + 1 value to the $loki property of the next object you insert in a collection. You can check the value of the last id by checking collection.maxId and deduce the next id by simply adding 1 to that.
Related
Prob- I want to query saved data from datastore as
id - [1,2,3]
2ndid - something,
Solution- It must get all the records for id-[1,2,3] where 2ndid - something gets match,
basically I want to filter based on 2ndId-something and want to get all records for the item which gets matched from id array.
ex- Table
id 2ndid
1 something
2 something
after query i must get records of id[1,2] as id[3] is not present
I tried for single id and single 2nd id, it worked but not working for array.
I'm using Flutter and Firestore, and I want to be able to create documents with an assigned ID field inside them, but I don't know how to make that the new document IDs Field is equal to the last document ID field number + 1.
For Example, if I have this field inside a document, I want to make the next document's correlativeNumber equal to this one + 1 = 87
The best option that you have is to store the last correlativeNumber into a document. Each time a new document is added, increment that number by 1. In this way, you can always know which number was used previously.
But there is something that should take into consideration. When it comes to document IDs, according to the official documentation:
Do not use monotonically increasing document IDs such as:
Customer1, Customer2, Customer3, ...
Product 1, Product 2, Product 3, ...
Such sequential IDs can lead to hotspots that impact latency.
So it's best to use the Firestore's built-in identifiers, which are by definition completely unique.
Is there a way I can find the last inserted document and the field, i.e. _id or id such that I can increment and use when inserting a new document?
The issue is that I create my own id count, but I do not store this, now I've deleted records, I cannot seem to add new records because I am attempting to use the same id.
There is no way to check insertion order in MongoDB, because the database does not keep any metadata in the collections regading the documents.
If your _id field is generated server-side then you need to have a very good algorithm for this value in order to provide collision avoidance and uniqueness while at the same time following any sequential constraints that you might have.
I have problem whit long Mongo find results. Example how can i start query starting from _id X
to forward Example I know I have document where is 1000 users details I know there is user called Peter in list I can make query Users.find({userName: "Peter"}) and get this on user _id but how I can get all users also after this with out I need return JSON from above "Peter"
With the little amount of information you have given, You need to do this in two steps:
Get the id of the first record that matches the name "peter".
db.test.findOne({"userName":"Peter"},{"_id":1});
Returns one document that satisfies the specified query criteria. If
multiple documents satisfy the query, this method returns the first
document according to the natural order which reflects the order of
documents on the disk. In capped collections, natural order is the
same as insertion order.
Once you have the id of the record with peter, you can retrieve the records with their id > the id of this record.
db.test.find({"_id":{$gte:x}});
Where, x is the id of the first record returned by the first query.
I have mongodb collection users.
Each user have field called rating which is between 1 and 5. It means that when user votes on another user he 'gives' him his vote which is a number between 1 and 5. I have a problem with storing this data in mongo document beacause I have to query user collection by rating field and I have to update it atomicly...
If I store both rating and number of votes when I can update votes_number with $inc operator but I cant atomicly set rating = ((rating*votes) + vote_val)/(votes+1)
I could just keep sum of votes and votes number in document and update both using $inc but then I cant query like WHERE votes_sum/votes_num > 3...
Is there any solution to this problem?
What you can do is use option two from above and then combine it with a cached result field. You can set up the data flow so the result field remains consistent with the rest of the document by using the filter predicate on your update.
Step one is to add a new field to your schema which will be your cached rating field. This will allow you to perform your range query without having to do the dynamic division. The problem you'll run into there is that you can't atomically increment the votes_sum & votes_num fields AND in the same atomic operation set the cached rating fields. So here's what you do.
1) Atomically increment the votes_sum and votes_num fields
2) Grab the _id, votes_sum & votes_num for the updated document
3) Update the rating but, as part of the filter predicate, include the _id, expected votes_sum and expected votes_num fields.
db.collection.update({_id: $id, votes_sum: $votes_sum, votes_num: $votes_num}, {$set: {rating: $votes_sum / $votes_num}});
This will ensure that nothing has changed since you updated the doc. If someone else comes along and updates those fields in between you updating them and generating the rating then the doc will not be returned in the find part of the update statement and thus it will not be updated with stale data
This pattern takes advantage of the fact that writes are atomic at the document level in MongoDB so you don't have to worry about the consistency of data within a document. The nice thing is that the rating will be set correctly because every operation to update the votes_sum and votes_num fields is followed by an update to rating.
See here for some sample code: http://docs.mongodb.org/manual/tutorial/isolate-sequence-of-operations/