use <databasename> will set a variable db to be the database
specified by <databasename>, so the database can be referred to
the variable db.
I wonder if a collection or document can also be
referred to by a variable, and if yes, how?
Every object in a MongoDB server has an identifier _id. If I am correct, a database, a collection and a document are objects.
How are the identifier of an object used in practice?
Both a database and a collection have a name. So we can refer to a
collection via its name e.g. mydb.mycollection.
Does a document also have a name?
Thanks.
A collection consists of several documents so a document as such do not have any name. _id distinguishes the documents. To fetch any particular document, you can filter it on the basis of _id or the data stored in it.
Referencing the db and then the collection in selected db will give you the required document.
Related
I want to retrieve a document from database to update it, so I need the document ID, how to get this ID if I didn't create the document with a custom ID (I've used database.createDocument() instead of database.documentWithID("docId"))
Thanks
If you haven't created the document with a custom Id then you can use
some other uniquely identifying property (or properties) of the document to query for the specific document.
Instead of querying and iterating over entire list of documents to find the match, you can create a view with the specified property/properties as index. You can then query for that view with startKey / endKey set to the desired property value. That will return the document of interest.
Alternatively,if it's an option, create the document with a custom ID.
We are planning on using MongoDB _id as a key that we would provide to the client. Therefore, the requirement is that this key should not change if we ever need to move the data from one collection to another. The copy will be performed using db.copyDatabase() or mongoimport.
One of the ways in which data can be copied from one collection to another is iterating through the documents in the first collection(C1) and inserting these documents in the second collection(C2). In this case _id should remain the same(in C2) because it would be present in the documents(of C1) being inserted(same as the case in which we would provide an _id ourselves).
However, if there is an alternate way in which documents are copied, the _id might change since it depends on :
(1) The UNIX timestamp
(2) Machine identifier
(3) ProcessId
(**This should only happen if MongoDB while copying removes _id from documents in C1 and regenerated them while inserting into C2?)
We want the _id values to be same irrespective of the location of the destination collection:
(1)within same database
(2)different database - same machine
(3)different database - different machine)
Thanks
No, the _id numbers will not change.
A new ObjectId is generated when a document without an _id field is inserted into the database. When you insert a document which already has an _id field, MongoDB won't touch it.
The timestamp, machine identifier and processID refer to those where the ObjectID was generated. This can be a database server, but it can also be generated by the MongoDB driver on the application server. In that case MongoDB will not change it on its own.
By the way: The _id can be an auto-generated ObjectId, but it doesn't have to. You can also use any other value as _id, as long as you can guarantee that it's unique. So when your data already has a natural key, you can use this as _id when you want to.
I have an ElasticSearch instance indexing a MongoDB database using the river by richardwilly98
There are two types of documents that are indexed:
documents referencing users
documents representing users
When these objects are added to mongodb richardwilly98's river generates something like the following:
document = {'user': {"$id" :
"5159a004c87126641f4f9530" } }
user_document = {'_id':"5159a004c87126641f4f9530",'username':'bob'}
If I perform a search for 'bob' i'd like any documents that reference the bob document to be returned. At the moment this doesn't happen because the username field is not related to the referencing documents in anyway.
Is it possible to do this? Does ElasticSearch have object references?
Thanks - let me know if I haven't been clear.
If each document belong to no more than one user, you can index documents as children of users. Then you can use has_parent filter to perform the search. However, if a single document can belong to more than one user, you will have to perform search in two steps. First you would have to find the user and then issue another search to find documents.
Elasticsearch supports parent field [1]. MongoDB river supports custom mapping [2] so _parent can now be used.
http://www.elasticsearch.org/guide/reference/mapping/parent-field/
https://github.com/richardwilly98/elasticsearch-river-mongodb/issues/64
I currently have a collection of small documents. Each document has an indexed geospacial field and *the default _id is never used in any query*. There will never be more than one document related to a particular geo location. I think it makes sense to override the default _id, and use the geospacial data for this somehow.
Question is, how do you use geospacial data as the unique id? Is it a case of creating a flat string from the geo field? E.g. 'x123456y123456'?
The _id field is the unique identifier for each document and thus is a needed field. The _id field is generated on document creation automatically if one is not provided. If you can provide this geospaital value when creating the document you should be able to use the string as you suggested, you cannot use an array as the _id value. However please be aware that once a document is created the _id becomes unchangeable. This means that using the _id field as a meaningful index of geospatial data may not be of much value.
Have a look here for more info on the _id field and here for some information about creating geospatial indexes in Mongo
I'm building a database with several collections. I have unique strings that I plan on using for all the documents in the main collection. Documents in other collections will reference documents in the main collection, which means I'll have to save said id's in the other collections. However, if _id's only need to be unique across a collection and not across an entire database, then I would just make the _id's in the other collections also use the aforementioned unique strings.
Also, I assume that in order to set my own _id's, all I have to do is have an "_id":"unique_string" property as part of the document that I insert, correct? I wouldn't need to convert the "unique_string" into another format, right?
Also, hypothetically speaking, would I be able to have a variable save the string "_id" and use that instead? Just to be clear, something as follows: var id = "_id" and then later on in the code (during an insert or a query for example) have id:"unique_string".
Best, and thanks,Sami
_ids have to be unique in a collection. You can quickly verify this by inserting two documents with the same _id in two different collections.
Your other assumptions are correct, just try them and see whether they work (they will). The proof of the pudding is in the eating.
Note: use _id directly, var id = "_id" just compilcates the code.