how mongodb store and manage the metadata?
for example, i want to know how many field in a collection, or how many collections in a db.
i can use show collections to show the collections but i don't know how it store in the mongodb, as a document?
how mongodb store and manage the metadata?
for example, i want to know how many field in a collection, or how many collections in a db.
i can use show collections to show the collections but i don't know how it store in the mongodb, as a document?
MongoDB stores its collections on files in the data folder configured.
The format of data in collections is BSON (Binary JSON).
MongoDB is document-oriented, so each document may have different fields at all. Each document is a well-formatted JSON, then the document has metadata stored inside itself.
For example:
{
"_id":{"$oid":"5fbcec5b0f751ebbc4d46516"},
"health":"green",
"status":"open",
"pri":1,
"rep":2
}
You have the field's metadata stored in the document itself.
The type of field is derived like JSON is: With quotes is a string, without quotes is a number.
Internally, Date objects are stored as a signed 64-bit integer representing the number of milliseconds since the Unix epoch (Jan 1, 1970).
You can write a Date object using the new Date(string) method.
Related
I am writing a program that is pulling all user data from my system and storing all the information in a single MongoDB collection. I am doing this each day and want to compare today's collection to yesterday's and see if any users were added/deleted/modified.
I have seen there is a Mongo command to compare, but that returns true/false. I am looking for a report of all changes.
mongodb always stores records in such a way that
{
'_id' : '1' ,
'name' : 'nidhi'
}
But i want to store in a way like
{ 123 , 'nidhi'}
I do not want to store keys again and again in database.
Is it possible with mongodb or with any other database.
Is there anything like sql is possible in nosql that I set the architecture first like mysql and then start inserting values in documents.
That is not possible with MongoDB. Documents are defined by Key/Value pairs. That has something to do that BSON (Binary JSON) – the internal storage format of MongoDB – was developed from JSON (JavaScript Object Notation). And without keys, you couldn't query the database at all, except by rather awkward positional parameters.
However, if disk space is so precious, you could revise your modeling to sth like:
{ _id:1,
values:['nidhi','foo','bar','baz']
}
However, since disk space is relatively cheap when compared to computing power (not a resource MongoDB uses a lot, though) and RAM (rule of thumb for MongoDB: the more, the better), you approach doesn't make sense. For a REST API to return a record to the client, all you have to do is (pseudo code):
var docToReturn = collection.findOne({_id:requestedId});
if(docToReturn){
response.send(200,docToReturn);
} else {
response.send(404,{'state':404,'error':'resource '+requestedId+' not available'});
}
Even if the data was possible to query with your approach, you would have to map the returned values to meaningful keys. And how would you deal with the fact that Arrays don't have a guaranteed order? Or with the fact that MongoDB has dynamic schemas, so that one doc in the collection may have a totally different structure than the other?
I am currently developing a Facebook application that calls upon the API and retrieve the user's info and display them on the front end.
However, since there are so many fields in the data. Could i actually store them into an array without knowing the fields using MongoDB?
Data in MongoDB has a flexible schema. Unlike SQL databases, where you
must determine and declare a table’s schema before inserting data,
MongoDB’s collections do not enforce document structure.
Read this articel: Data Modeling Introduction
Instead of inserting data to array you can insert it as a new key-value pair to document.
I am developing a system where items can be shared with other users via an access key. I'm storing the access keys as fields within a shareinfo object (embedded within the item's document), as shown below:
shareinfo:{
........
<nth key>: <permissions object - may be complex and large>
........
}
When an item is accessed I check shareinfo.key and find if its valid or not.
Currently, to list the keys I am loading (in Java) the entire shareinfo object in memory and running keySet() on it to retrieve and return the keys while the rest of the data is wasted.
Here's the problem: I want to get the list of keys (i.e. object field names) without the accompanying data (because in some cases the permissions object is noticeably large).
I could not find any query in the mongodb docs for such a query. I want to know whether it's possible or not? Or is there an optimized way to load the list of field names into the application without the accompanying field values?
I had the same issue when trying to understand the structure of an existing mongodb database using the mongodb shell. Then I found that I can use the JavaScript function Object.keys() to retrieve an array of the object fields. (Tested with MongoDb 2.4.2)
Object.keys(db.collection.findOne())
MongoDB has a schema-less design, which means that any document could have different fields contained within it from any other document. To get an exhaustive list of all the fields for all the documents, you would need to traverse all the documents in the collection and enumerate each field. For any reasonably sized collection this is going to be an extremely expensive operation.
There are a couple of helpers that make this more simple: Variety and schema.js . They both allow you to limit the documents inspected, and they report on coverage as well. Both amount to doing a map/reduce on the collections, but are at least simpler than having to write one yourself.
If you are just looking for top-level fields across all documents (don't care about sub-fields and all the details statistics provided by variety)
Here is the one-liner
db.things.aggregate([{$project: {arrayofkeyvalue: {$objectToArray: "$$ROOT"}}}, {$unwind:"$arrayofkeyvalue"}, {$group:{_id:null, allkeys:{$addToSet:"$arrayofkeyvalue.k"}}}])
Consider the following scenario with MongoDB:
Three writers (A,B,C) insert a document into the same collection.
A inserts first, followed by B, followed by C.
How can we guarantee A retrieves the ObjectId of the document he inserted and not B's document or C's document? Do we need to serialize the writes (i.e., only permit B to write after A inserts and retrieves the ObjectId), or does MongoDB offer some native functionality for this scenario?
Thanks!
We're on Rails.
the normal pattern here is for the driver to allocate the ObjectId and then you know what it is for the insert even before the server gets it.
You can generate the _id value in your client applications (writers) before inserting the document. This way you don't need to rely on the server generating the ObjectId an retrieving the correct value. Most MongoDB language drivers will do this for you automatically if you leave the _id blank.