Is there a way to get an array slice at the mongo db level? I am trying to do something similar to the following: Model.find({filter: option}, startindex, endindex). Currently the only option I found is to do the following:
let result = await Model.find({filter: option});
returh result.slice(startIndex, endIndex)
Unfortunately, this does not work since I have to pull the full record each time. If I can do this at the mongo level that would be great. Thank you for your help!
UPDATE:
After further research I found a possible solution:
Model.find({filter: option}).skip(skip).limit(limit);
it seems with this method I am able to do slice the document array in the mongo db. If you have any other ideas please let me know. Thank you!
from what i know, there isnt a way to get an slice of an array from an document, but there is the select
PS: skip skips the first documents found by the query, and limit limits the amount returned by the query
Recently I try to use MongoDB-Connector from the Official documents
JavaMongoRDD<Document> rdd = MongoSpark.load(jsc);
but the demo will load all the data from my collection ,
I just want to run this command in my java or scala code
db.pointer.find({"inserttime":{$lt:new Date(2018,0,4,7,0,10),$gte:new Date(2018,0,4,7,0,0)}},{"inserttime":1,})
I know I can use RDD.filter() to get the data I want.
but it will query all data at the first time ,That's not what I want.
Thanks in advance .
EDIT:
Did it provided any method with condition query to reduce the result data when query .
like JDBC:
find(and(eq("status", "A"),or(lt("qty", 30), regex("item", "^p"))));
The documentation uses aggregations to filter data at the database level, so you can do the same.
// the following example was not tested
BasicDBObject query = new BasicDBObject("$lt", new Date(2018,0,4,7,0,10);
query.put("$gte", new Date(2018,0,4,7,0,0));
JavaMongoRDD<Document> aggregatedRdd = rdd.withPipeline(singletonList(query));
Date range query trick came from this answer
I have been trying to sort documents in Mongo collection by ObjectId in descending order from Pymongo and use the last added record using this code
record = collection.find({}).sort('_id', -1).limit(1)
However, I am getting 'expected a character buffer object'error. Not sure what is happening here.
Also, if anyone can think of a better way to get the last added record from mongo collection using Pymongo, it will be great help.
for item in collection.find().sort("_id", pymongo.DESCENDING).limit(1):
# do something with your item.
This error indicates to me that your "collection" variable holds a string, not a PyMongo Collection instance. I need to see the code that sets the "collection" variable to know what mistake you're making. But let's start from the top:
import pymongo
client = pymongo.MongoClient()
collection = client.db
cursor = collection.find({}).sort('_id', -1).limit(1)
record = cursor.next()
This will get you a recently-added document, but not always the most recently added. The timestamp portion of an ObjectId is only precise to one second, and since it's generated on the client machine that inserts the document, ObjectId is subject to clock skew.
If and only if the collection is capped, you can reliably get the last document like:
cursor = collection.find({}).sort('$natural', -1).limit(1)
record = cursor.next()
Such as to obtain a post before an after a record time field created
Try to use the following statement to obtain articles
# Created is the time of the creation of the current article
# Before a
prev_post = db.Post.find ({'created': {'$ lt': created}}, sort = [('created', -1)], limit = 1)
# After a
next_post = db.Post.find ({'created': {'$ gt': created}}, sort = [('created', 1)], limit = 1)
The result turn to be discontinuous,sometimes skip several records.I don't know why,maybe I misunderstand the FIND?
Help please.
It may seem a strange behaviour indeed, but MongoDB does not guarantee you the order of stored records, unless you're querying an array (in which records are kept in the insertion order). I believe what MongoDB does - it reaches the first document that matches your query and returns it.
Bottomline: if the logic requires neighbour records, use arrays.
I think it's your created. the time type has a problem ,if the record time is same ,the system don't know which is the choosing one.you can use _id , have a try.
So this is Day 3 of learning Mongo Db. I'm coming from the MySql universe...
A lot of times when I need to write a query for a MySql table I'm unfamiliar with, I would use the "desc" command - basically telling me what fields I should include in my query.
How would I do that for a Mongo db? I know, I know...I'm searching for a schema in a schema-less database. =) But how else would users know what fields to use in their queries?
Am I going at this the wrong way? Obviously I'm trying to use a MySql way of doing things in a Mongo db. What's the Mongo way?
Type the below query in editor / mongoshell
var col_list= db.emp.findOne();
for (var col in col_list) { print (col) ; }
output will give you name of columns in collection :
_id
name
salary
There is no good answer here. Because there is no schema, you can't 'describe' the collection. In many (most?) MongoDb applications, however, the schema is defined by the structure of the object hierarchy used in the writing application (java or c# or whatever), so you may be able to reflect over the object library to get that information. Otherwise there is a bit of trial and error.
This is my day 30 or something like that of playing around with MongoDB. Unfortunately, we have switched back to MySQL after working with MongoDB because of my company's current infrastructure issues. But having implemented the same model on both MongoDB and MySQL, I can clearly see the difference now.
Of course, there is a schema involved when dealing with schema-less databases like MongoDB, but the schema is dictated by the application, not the database. The database will shove in whatever it is given. As long as you know that admins are not secretly logging into Mongo and making changes, and all access to the database is controller through some wrapper, the only place you should look at for the schema is your model classes. For instance, in our Rails application, these are two of the models we have in Mongo,
class Consumer
include MongoMapper::Document
key :name, String
key :phone_number, String
one :address
end
class Address
include MongoMapper::EmbeddedDocument
key :street, String
key :city, String
key :state, String
key :zip, String
key :state, String
key :country, String
end
Now after switching to MySQL, our classes look like this,
class Consumer < ActiveRecord::Base
has_one :address
end
class Address < ActiveRecord::Base
belongs_to :consumer
end
Don't get fooled by the brevity of the classes. In the latter version with MySQL, the fields are being pulled from the database directly. In the former example, the fields are right there in front of our eyes.
With MongoDB, if we had to change a particular model, we simply add, remove, or modify the fields in the class itself and it works right off the bat. We don't have to worry about keeping the database tables/columns in-sync with the class structure. So if you're looking for the schema in MongoDB, look towards your application for answers and not the database.
Essentially I am saying the exactly same thing as #Chris Shain :)
While factually correct, you're all making this too complex. I think the OP just wants to know what his/her data looks like. If that's the case, you can just
db.collectionName.findOne()
This will show one document (aka. record) in the database in a pretty format.
I had this need too, Cavachon. So I created an open source tool called Variety which does exactly this: link
Hopefully you'll find it to be useful. Let me know if you have questions, or any issues using it.
Good luck!
AFAIK, there isn't a way and it is logical for it to be so.
MongoDB being schema-less allows a single collection to have a documents with different fields. So there can't really be a description of a collection, like the description of a table in the relational databases.
Though this is the case, most applications do maintain a schema for their collections and as said by Chris this is enforced by your application.
As such you wouldn't have to worry about first fetching the available keys to make a query. You can just ask MongoDB for any set of keys (i.e the projection part of the query) or query on any set of keys. In both cases if the keys specified exist on a document they are used, otherwise they aren't. You will not get any error.
For instance (On the mongo shell) :
If this is a sample document in your people collection and all documents follow the same schema:
{
name : "My Name"
place : "My Place"
city : "My City"
}
The following are perfectly valid queries :
These two will return the above document :
db.people.find({name : "My Name"})
db.people.find({name : "My Name"}, {name : 1, place :1})
This will not return anything, but will not raise an error either :
db.people.find({first_name : "My Name"})
This will match the above document, but you will have only the default "_id" property on the returned document.
db.people.find({name : "My Name"}, {first_name : 1, location :1})
print('\n--->', Object.getOwnPropertyNames(db.users.findOne())
.toString()
.replace(/,/g, '\n---> ') + '\n');
---> _id
---> firstName
---> lastName
---> email
---> password
---> terms
---> confirmed
---> userAgent
---> createdAt
This is an incomplete solution because it doesn't give you the exact types, but useful for a quick view.
const doc = db.collectionName.findOne();
for (x in doc) {
print(`${x}: ${typeof doc[x]}`)
};
If you're OK with running a Map / Reduce, you can gather all of the possible document fields.
Start with this post.
The only problem here is that you're running a Map / Reduce on which can be resource intensive. Instead, as others have suggested, you'll want to look at the code that writes the actual data.
Just because the database doesn't have a schema doesn't mean that there is no schema. Generally speaking the schema information will be in the code.
I wrote a small mongo shell script that may help you.
https://gist.github.com/hkasera/9386709
Let me know if it helps.
You can use a UI tool mongo compass for mongoDb. This shows all the fields in that collection and also shows the variation of data in it.
If you are using NodeJS and want to get the all the field names using the API request, this code works for me-
let arrayResult = [];
db.findOne().exec(function (err, docs)){
if(err)
//show error
const JSONobj = JSON.parse(JSON.stringify(docs));
for(let key in JSONobj) {
arrayResult.push(key);
}
return callback(null, arrayResult);
}
The arrayResult will give you entire field/ column names
Output-
[
"_id",
"emp_id",
"emp_type",
"emp_status",
"emp_payment"
]
Hope this works for you!
Consider you have collection called people and you want to find the fields and it's data-types. you can use below query
function printSchema(obj) {
for (var key in obj) {
print( key, typeof obj[key]) ;
}
};
var obj = db.people.findOne();
printSchema(obj)
The result of this query will be like below,
you can use Object.keys like in JavaScript
Object.keys(db.movies.findOne())