i would like to get a collection of documents with a specific one included (i have the _id field of this document).
The difficulty is to also get the 3 documents which precede and follow this document (in the inserting order).
I really doubt it is possible in one request because if i use the where rule to match the id of the document i want to get, i don't know how i could also match the other ones.
So i know i could do one find for the document and get the 3 oldest and newest with another find refering the date of the created_at field, but i am interesting to know if i can improve this treatment.
Related
Looking for a way to view the results of a query where the displayed fields in the doc are ordered (lexicographically in my case).
Example:
I'm getting back from a query one document, which is what I need. This document has 30 fields and I'm looking to see the value in one of them. My issue is that the order of the fields is, well, kinda random. Not sorted in any way I'm aware of.
I'm using this php package to make queries - https://github.com/jenssegers/laravel-mongodb
The situation is, there are two fields, user_id and post_status among others. I want to retrieve all the documents in that collection, but when post_status field value is draft, that should be retrieved only when user_id is a given string. The idea is, only logged in user finds their drafted posts among other posts.
I'm having hard time finding any solution for this problem. The app is still not in production. If I should store data is some different manner, that is an option as well.
Find documents with a certain field value only when another field value is a given string
The question your are framing is simply convert into a and query, how let's see it
when another field value is a given string
This means that you have some result sets and you need to filter out when user_id match with some string. i.e some result sets and user_id = <id>
Now consider the first part of the sentence Find documents with a certain field value
This means you are filtering the records with some values i.e "status" = "draft" and whatever result will come and want again to filter on the basis of user_id = <id>
So finally you will end-up with below query:
db.collectionName.find({"status":"draft", "user_id": "5c618615903aaa496d129d90"})
Hope this explanation will help you out or you can rephrase your question I will try to modify by ans.
I would like to encode some meaning behinds first N characters of every document ID i.e. make first three characters determine a document type sensible to the system being used in.
You can have a custom _id when you insert the document. If the document to be inserted doesn't contain _id, then MongoDB will insert a ObejctId for you.
The _id can be of any type but keeping it uniform for all the documents makes sense if you are accessing from application layer.
You can refer one of the old questions at SO - How to generate unique object id in mongodb
I have a MongoDB collection as follows:
comment_id (number)
comment_title (text)
score (number)
time_score (number)
final_score (number)
created_time (timestamp)
Score is and integer that's usually updated using $inc 1 or -1 whenever someone votes up or down for that record.
but time_score is updated using a function relative to timestamp and current time and other factors like how many (whole days passed) and how many (whole weeks passed) .....etc
So I do $inc and $dec on db directly but for the time_score, I retrieve data from db calculate the new score and write it back. What I'm worried about is that in case many users incremented the "score" field during my calculation of time_score then when I wrote time_score to db it'll corrupt the last value of score.
To be more clear does updating specific fields in a record in Mongo rewrites the whole record or only the updated fields ? (Assume that all these fields are indexed).
By default, whole documents are rewritten. To specify the fields that are changed without modifying anything else, use the $set operator.
Edit: The comments on this answer are correct - any of the update modifiers will cause only relevant fields to be rewritten rather than the whole document. By "default", I meant a case where no special modifiers are used (a vanilla document is provided).
The algorithm you are describing is definitely not thread-safe.
When you read the entire document, change one field and then write back the entire document, you are creating a race condition - any field in the document that is modified after your read but before your write will be overwritten by your update.
That's one of many reasons to use $set or $inc operators to atomically set individual fields rather than updating the entire document based on possibly stale values in it.
Another reason is that setting/updating a single field "in-place" is much more efficient than writing the entire document. In addition you have less load on your network when you are passing smaller update document ({$set:{field:value}}, rather than entire new version of the document).
I have collections with huge amount of Documents on which I need to do custom search with various different queries.
Each Document have boolean property. Let's call it "isInTop".
I need to show Documents which have this property first in all queries.
Yes. I can easy do sort in this field like:
.sort( { isInTop: -1 } );
And create proper index with field "isInTop" as last field in it. But this will be work slowly, as indexes in mongo works best with unique fields.
So is there is solution to show Documents with field "isInTop" on top of each query?
I see two solutions here.
First: set Documents wich need to be in top the _id from "future". As you know, ObjectId contains timestamp. So I can create ObjectId with timestamp from future and use natural order
Second: create separate collection for Ducuments wich need to be in top. And do queries in it first.
Is there is any other solutions for this problem? Which will work fater?
UPDATE
I have done this issue with sorting on custom field which represent rank.
Using the _id field trick you mention has the problem that at some point in time you will reach the special time, and you can't change the _id field (without inserting a new document and removing the old one).
Creating a special collection which just holds the ones you care about is probably the best option. It gives you the ability to logically (and to some extent, physically) separate the documents.
Newly introduced in mongodb there is also support for a "sparse" index which may fulfill your needs as well. You could only set the "isInTop" field when you want it to be special, and then create a sparse index on it which would not have the problems you would normally have with a single indexed boolean field (in btrees).