MongoDB + Node.js: unable to insert date properly - mongodb

I′ve been using node-mongoskin to connect this two. Everything was ok until I queried some "date" field which I think should be returned as javascript′s Date object. But result′s type was string, which is odd (for me) and inconvenient.
Inserting looks something like this:
var doc = {
date: new Date(),
info: 'Some info'
}
db.users.insert( doc, {safe: true}, function(err, res) {
...
});
And result of above is (without _id field):
{ "date" : "Mon Oct 24 2011 18:00:57 GMT+0400 (MSK)", "info": "Some info" }
However, inserting with MongoDB Shell works just fine, except type of field is ISODate
> db.things.insert({ date: new Date() }); db.things.find();
{ "_id" : ObjectId("4eae9f2a34067b92db8deb40"), "date" : ISODate("2011-10-31T13:14:18.947Z") }
So, the question is: how should I insert documents to query date fields as Date object? What I want is setting fields on database-server-side. I just send something like null-fields, and db-server setting those for me using default mongo′s mechanisms.
Inserting timestamps (as native MongoDB timestamp) is also a problem, but it′s not such a big deal.
PS: No luck going through mongoskin and mongodb-native docs.


It was probably some bug in my code or the mongo driver. Now, the following works just fine:
db.collection.insert({d: new Date()});
Timestamps support described here: http://mongodb.github.com/node-mongodb-native/api-bson-generated/timestamp.html.


ISODate is the native way for mongo to store date. I use node-mongodb-native npm module and I store/retrieve javascript Date using new Date() idiom like in your examples. I don't know if it's a recent correction because I started node and Mongo in 2012, but using date was pretty straightforward for me.


JavaScript code:
collection.insert({"className" : "models.Action",
"title" : "Email",
"description" : "How are you today?",
"creationDate" : new Date("Fry, 4 May 2012 10:30:08 +0200 (CEST)"),
"creator" : dbref },
produced in mongoDB
db.action.find({"title":"Email"})
> db.action.find({"title":"Email"})
{ "className" : "models.Action", "title" : "Email", "description" : "How are you today?", "creationDate" : ISODate("2012-05-04T08:30:08Z"), "creator" : { "$ref" : "person", "$id" : ObjectId("4f995e4824ac8d68f63adf69") }, "_id" : ObjectId("4fa79e2e92c2a19a09000002") }

Related

convert timestamp to ISO date in Mongodb version 2.4

I have mongodb version 2.4.10 installed. The structure of the document is :-
{
"_id" : ObjectId("5d15f245f4dda1e055091ae1"),
"name" : "test site service",
"starFromTimestamp" : NumberLong(1559275200),
"toTimestamp" : NumberLong(1561867200),
"uuid" : "ssg-5d15f245f2893825813309"
}
I excuted the following code in Mongo shell in order to convert timestamp to ISODate
db.servicesitegroup.find().forEach(function(doc) {
doc.startISODate=new Date(doc.starFromTimestamp);
db.servicesitegroup.save(doc);
})
The document got updated and the resultset looks like:-
{
"_id" : ObjectId("5d15f245f4dda1e055091ae1"),
"name" : "test site service",
"starFromTimestamp" : NumberLong(1559275200),
"toTimestamp" : NumberLong(1561867200),
"uuid" : "ssg-5d15f245f2893825813309",
"startISODate" : ISODate("1970-01-19T01:07:55.200Z")
}
If I use the timestamp converter, then the value of 1559275200 amounts to Friday, May 31, 2019 4:00:00 AM . Why is the timestamp not being converted to the correct value? Can anyone guide me here.

I needed to multiply the timestamp value with 1000 .
db.servicesitegroup.find().forEach(function(doc) {
doc.startISODate=new Date(doc.starFromTimestamp * 1000);
db.servicesitegroup.save(doc);
})
The code above gives me the correct output.

MongoDB aggregation via db.runCommand(): $match by date [duplicate]

I've been playing around storing tweets inside mongodb, each object looks like this:
{
"_id" : ObjectId("4c02c58de500fe1be1000005"),
"contributors" : null,
"text" : "Hello world",
"user" : {
"following" : null,
"followers_count" : 5,
"utc_offset" : null,
"location" : "",
"profile_text_color" : "000000",
"friends_count" : 11,
"profile_link_color" : "0000ff",
"verified" : false,
"protected" : false,
"url" : null,
"contributors_enabled" : false,
"created_at" : "Sun May 30 18:47:06 +0000 2010",
"geo_enabled" : false,
"profile_sidebar_border_color" : "87bc44",
"statuses_count" : 13,
"favourites_count" : 0,
"description" : "",
"notifications" : null,
"profile_background_tile" : false,
"lang" : "en",
"id" : 149978111,
"time_zone" : null,
"profile_sidebar_fill_color" : "e0ff92"
},
"geo" : null,
"coordinates" : null,
"in_reply_to_user_id" : 149183152,
"place" : null,
"created_at" : "Sun May 30 20:07:35 +0000 2010",
"source" : "web",
"in_reply_to_status_id" : {
"floatApprox" : 15061797850
},
"truncated" : false,
"favorited" : false,
"id" : {
"floatApprox" : 15061838001
}
How would I write a query which checks the created_at and finds all objects between 18:47 and 19:00? Do I need to update my documents so the dates are stored in a specific format?

Querying for a Date Range (Specific Month or Day) in the MongoDB Cookbook has a very good explanation on the matter, but below is something I tried out myself and it seems to work.
items.save({
name: "example",
created_at: ISODate("2010-04-30T00:00:00.000Z")
})
items.find({
created_at: {
$gte: ISODate("2010-04-29T00:00:00.000Z"),
$lt: ISODate("2010-05-01T00:00:00.000Z")
}
})
=> { "_id" : ObjectId("4c0791e2b9ec877893f3363b"), "name" : "example", "created_at" : "Sun May 30 2010 00:00:00 GMT+0300 (EEST)" }
Based on my experiments you will need to serialize your dates into a format that MongoDB supports, because the following gave undesired search results.
items.save({
name: "example",
created_at: "Sun May 30 18.49:00 +0000 2010"
})
items.find({
created_at: {
$gte:"Mon May 30 18:47:00 +0000 2015",
$lt: "Sun May 30 20:40:36 +0000 2010"
}
})
=> { "_id" : ObjectId("4c079123b9ec877893f33638"), "name" : "example", "created_at" : "Sun May 30 18.49:00 +0000 2010" }
In the second example no results were expected, but there was still one gotten. This is because a basic string comparison is done.

To clarify. What is important to know is that:
Yes, you have to pass a Javascript Date object.
Yes, it has to be ISODate friendly
Yes, from my experience getting this to work, you need to manipulate the date to ISO
Yes, working with dates is generally always a tedious process, and mongo is no exception
Here is a working snippet of code, where we do a little bit of date manipulation to ensure Mongo (here i am using mongoose module and want results for rows whose date attribute is less than (before) the date given as myDate param) can handle it correctly:
var inputDate = new Date(myDate.toISOString());
MyModel.find({
'date': { $lte: inputDate }
})

Python and pymongo
Finding objects between two dates in Python with pymongo in collection posts (based on the tutorial):
from_date = datetime.datetime(2010, 12, 31, 12, 30, 30, 125000)
to_date = datetime.datetime(2011, 12, 31, 12, 30, 30, 125000)
for post in posts.find({"date": {"$gte": from_date, "$lt": to_date}}):
print(post)
Where {"$gte": from_date, "$lt": to_date} specifies the range in terms of datetime.datetime types.

db.collection.find({"createdDate":{$gte:new ISODate("2017-04-14T23:59:59Z"),$lte:new ISODate("2017-04-15T23:59:59Z")}}).count();
Replace collection with name of collection you want to execute query

MongoDB actually stores the millis of a date as an int(64), as prescribed by http://bsonspec.org/#/specification
However, it can get pretty confusing when you retrieve dates as the client driver will instantiate a date object with its own local timezone. The JavaScript driver in the mongo console will certainly do this.
So, if you care about your timezones, then make sure you know what it's supposed to be when you get it back. This shouldn't matter so much for the queries, as it will still equate to the same int(64), regardless of what timezone your date object is in (I hope). But I'd definitely make queries with actual date objects (not strings) and let the driver do its thing.

Use this code to find the record between two dates using $gte and $lt:
db.CollectionName.find({"whenCreated": {
'$gte': ISODate("2018-03-06T13:10:40.294Z"),
'$lt': ISODate("2018-05-06T13:10:40.294Z")
}});

Using with Moment.js and Comparison Query Operators
var today = moment().startOf('day');
// "2018-12-05T00:00:00.00
var tomorrow = moment(today).endOf('day');
// ("2018-12-05T23:59:59.999
Example.find(
{
// find in today
created: { '$gte': today, '$lte': tomorrow }
// Or greater than 5 days
// created: { $lt: moment().add(-5, 'days') },
}), function (err, docs) { ... });

db.collection.find({$and:
[
{date_time:{$gt:ISODate("2020-06-01T00:00:00.000Z")}},
{date_time:{$lt:ISODate("2020-06-30T00:00:00.000Z")}}
]
})
##In case you are making the query directly from your application ##
db.collection.find({$and:
[
{date_time:{$gt:"2020-06-01T00:00:00.000Z"}},
{date_time:{$lt:"2020-06-30T00:00:00.000Z"}}
]
})

You can also check this out. If you are using this method, then use the parse function to get values from Mongo Database:
db.getCollection('user').find({
createdOn: {
$gt: ISODate("2020-01-01T00:00:00.000Z"),
$lt: ISODate("2020-03-01T00:00:00.000Z")
}
})

Save created_at date in ISO Date Format then use $gte and $lte.
db.connection.find({
created_at: {
$gte: ISODate("2010-05-30T18:47:00.000Z"),
$lte: ISODate("2010-05-30T19:00:00.000Z")
}
})

use $gte and $lte to find between date data's in mongodb
var tomorrowDate = moment(new Date()).add(1, 'days').format("YYYY-MM-DD");
db.collection.find({"plannedDeliveryDate":{ $gte: new Date(tomorrowDate +"T00:00:00.000Z"),$lte: new Date(tomorrowDate + "T23:59:59.999Z")}})

mongoose.model('ModelName').aggregate([
{
$match: {
userId: mongoose.Types.ObjectId(userId)
}
},
{
$project: {
dataList: {
$filter: {
input: "$dataList",
as: "item",
cond: {
$and: [
{
$gte: [ "$$item.dateTime", new Date(`2017-01-01T00:00:00.000Z`) ]
},
{
$lte: [ "$$item.dateTime", new Date(`2019-12-01T00:00:00.000Z`) ]
},
]
}
}
}
}
}
])

For those using Make (formerly Integromat) and MongoDB:
I was struggling to find the right way to query all records between two dates. In the end, all I had to do was to remove ISODate as suggested in some of the solutions here.
So the full code would be:
"created": {
"$gte": "2016-01-01T00:00:00.000Z",
"$lt": "2017-01-01T00:00:00.000Z"
}
This article helped me achieve my goal.
UPDATE
Another way to achieve the above code in Make (formerly Integromat) would be to use the parseDate function. So the code below will return the same result as the one above :
"created": {
"$gte": "{{parseDate("2016-01-01"; "YYYY-MM-DD")}}",
"$lt": "{{parseDate("2017-01-01"; "YYYY-MM-DD")}}"
}
⚠️ Be sure to wrap {{parseDate("2017-01-01"; "YYYY-MM-DD")}} between quotation marks.

Convert your dates to GMT timezone as you're stuffing them into Mongo. That way there's never a timezone issue. Then just do the math on the twitter/timezone field when you pull the data back out for presentation.

Why not convert the string to an integer of the form YYYYMMDDHHMMSS? Each increment of time would then create a larger integer, and you can filter on the integers instead of worrying about converting to ISO time.

Scala:
With joda DateTime and BSON syntax (reactivemongo):
val queryDateRangeForOneField = (start: DateTime, end: DateTime) =>
BSONDocument(
"created_at" -> BSONDocument(
"$gte" -> BSONDateTime(start.millisOfDay().withMinimumValue().getMillis),
"$lte" -> BSONDateTime(end.millisOfDay().withMaximumValue().getMillis)),
)
where millisOfDay().withMinimumValue() for "2021-09-08T06:42:51.697Z" will be "2021-09-08T00:00:00.000Z"
and
where millisOfDay(). withMaximumValue() for "2021-09-08T06:42:51.697Z" will be "2021-09-08T23:59:99.999Z"

i tried in this model as per my requirements i need to store a date when ever a object is created later i want to retrieve all the records (documents ) between two dates
in my html file
i was using the following format mm/dd/yyyy
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<script>
//jquery
$(document).ready(function(){
$("#select_date").click(function() {
$.ajax({
type: "post",
url: "xxx",
datatype: "html",
data: $("#period").serialize(),
success: function(data){
alert(data);
} ,//success
}); //event triggered
});//ajax
});//jquery
</script>
<title></title>
</head>
<body>
<form id="period" name='period'>
from <input id="selecteddate" name="selecteddate1" type="text"> to
<input id="select_date" type="button" value="selected">
</form>
</body>
</html>
in my py (python) file i converted it into "iso fomate"
in following way
date_str1 = request.POST["SelectedDate1"]
SelectedDate1 = datetime.datetime.strptime(date_str1, '%m/%d/%Y').isoformat()
and saved in my dbmongo collection with "SelectedDate" as field in my collection
to retrieve data or documents between to 2 dates i used following query
db.collection.find( "SelectedDate": {'$gte': SelectedDate1,'$lt': SelectedDate2}})

Cannot search mongo db by timestamp

Hey I am having an issue with this line of code
db.eventrecords.find({ timestamp: { $gte:ISODate("2013-11-19T14:00:00Z"),
$lt: ISODate("2018-11-19T20:00:00Z") } })
This query and any similiar query using timestamp returns nothing even though I know there is data there.
Here is an example of that data returned with d.eventrecords.find()
{ "_id" : ObjectId("5a0c9e22da6e704174881b6c"), "userId" :
"59e0265c387d7d22a4f81533", "timestamp" : "Wed Nov 15 2017 15:05:54 GMT-
0500 (Eastern Standard Time)", "name" : "LOGIN" }
Can anyone shed light on why this query won't return data?

You can't do that because you store dates as string (not timestamp).

Moongose MongoDB: LIKE operator in search for searching dates [duplicate]

I've been playing around storing tweets inside mongodb, each object looks like this:
{
"_id" : ObjectId("4c02c58de500fe1be1000005"),
"contributors" : null,
"text" : "Hello world",
"user" : {
"following" : null,
"followers_count" : 5,
"utc_offset" : null,
"location" : "",
"profile_text_color" : "000000",
"friends_count" : 11,
"profile_link_color" : "0000ff",
"verified" : false,
"protected" : false,
"url" : null,
"contributors_enabled" : false,
"created_at" : "Sun May 30 18:47:06 +0000 2010",
"geo_enabled" : false,
"profile_sidebar_border_color" : "87bc44",
"statuses_count" : 13,
"favourites_count" : 0,
"description" : "",
"notifications" : null,
"profile_background_tile" : false,
"lang" : "en",
"id" : 149978111,
"time_zone" : null,
"profile_sidebar_fill_color" : "e0ff92"
},
"geo" : null,
"coordinates" : null,
"in_reply_to_user_id" : 149183152,
"place" : null,
"created_at" : "Sun May 30 20:07:35 +0000 2010",
"source" : "web",
"in_reply_to_status_id" : {
"floatApprox" : 15061797850
},
"truncated" : false,
"favorited" : false,
"id" : {
"floatApprox" : 15061838001
}
How would I write a query which checks the created_at and finds all objects between 18:47 and 19:00? Do I need to update my documents so the dates are stored in a specific format?

Querying for a Date Range (Specific Month or Day) in the MongoDB Cookbook has a very good explanation on the matter, but below is something I tried out myself and it seems to work.
items.save({
name: "example",
created_at: ISODate("2010-04-30T00:00:00.000Z")
})
items.find({
created_at: {
$gte: ISODate("2010-04-29T00:00:00.000Z"),
$lt: ISODate("2010-05-01T00:00:00.000Z")
}
})
=> { "_id" : ObjectId("4c0791e2b9ec877893f3363b"), "name" : "example", "created_at" : "Sun May 30 2010 00:00:00 GMT+0300 (EEST)" }
Based on my experiments you will need to serialize your dates into a format that MongoDB supports, because the following gave undesired search results.
items.save({
name: "example",
created_at: "Sun May 30 18.49:00 +0000 2010"
})
items.find({
created_at: {
$gte:"Mon May 30 18:47:00 +0000 2015",
$lt: "Sun May 30 20:40:36 +0000 2010"
}
})
=> { "_id" : ObjectId("4c079123b9ec877893f33638"), "name" : "example", "created_at" : "Sun May 30 18.49:00 +0000 2010" }
In the second example no results were expected, but there was still one gotten. This is because a basic string comparison is done.

To clarify. What is important to know is that:
Yes, you have to pass a Javascript Date object.
Yes, it has to be ISODate friendly
Yes, from my experience getting this to work, you need to manipulate the date to ISO
Yes, working with dates is generally always a tedious process, and mongo is no exception
Here is a working snippet of code, where we do a little bit of date manipulation to ensure Mongo (here i am using mongoose module and want results for rows whose date attribute is less than (before) the date given as myDate param) can handle it correctly:
var inputDate = new Date(myDate.toISOString());
MyModel.find({
'date': { $lte: inputDate }
})

Python and pymongo
Finding objects between two dates in Python with pymongo in collection posts (based on the tutorial):
from_date = datetime.datetime(2010, 12, 31, 12, 30, 30, 125000)
to_date = datetime.datetime(2011, 12, 31, 12, 30, 30, 125000)
for post in posts.find({"date": {"$gte": from_date, "$lt": to_date}}):
print(post)
Where {"$gte": from_date, "$lt": to_date} specifies the range in terms of datetime.datetime types.

db.collection.find({"createdDate":{$gte:new ISODate("2017-04-14T23:59:59Z"),$lte:new ISODate("2017-04-15T23:59:59Z")}}).count();
Replace collection with name of collection you want to execute query

MongoDB actually stores the millis of a date as an int(64), as prescribed by http://bsonspec.org/#/specification
However, it can get pretty confusing when you retrieve dates as the client driver will instantiate a date object with its own local timezone. The JavaScript driver in the mongo console will certainly do this.
So, if you care about your timezones, then make sure you know what it's supposed to be when you get it back. This shouldn't matter so much for the queries, as it will still equate to the same int(64), regardless of what timezone your date object is in (I hope). But I'd definitely make queries with actual date objects (not strings) and let the driver do its thing.

Use this code to find the record between two dates using $gte and $lt:
db.CollectionName.find({"whenCreated": {
'$gte': ISODate("2018-03-06T13:10:40.294Z"),
'$lt': ISODate("2018-05-06T13:10:40.294Z")
}});

Using with Moment.js and Comparison Query Operators
var today = moment().startOf('day');
// "2018-12-05T00:00:00.00
var tomorrow = moment(today).endOf('day');
// ("2018-12-05T23:59:59.999
Example.find(
{
// find in today
created: { '$gte': today, '$lte': tomorrow }
// Or greater than 5 days
// created: { $lt: moment().add(-5, 'days') },
}), function (err, docs) { ... });

db.collection.find({$and:
[
{date_time:{$gt:ISODate("2020-06-01T00:00:00.000Z")}},
{date_time:{$lt:ISODate("2020-06-30T00:00:00.000Z")}}
]
})
##In case you are making the query directly from your application ##
db.collection.find({$and:
[
{date_time:{$gt:"2020-06-01T00:00:00.000Z"}},
{date_time:{$lt:"2020-06-30T00:00:00.000Z"}}
]
})

You can also check this out. If you are using this method, then use the parse function to get values from Mongo Database:
db.getCollection('user').find({
createdOn: {
$gt: ISODate("2020-01-01T00:00:00.000Z"),
$lt: ISODate("2020-03-01T00:00:00.000Z")
}
})

Save created_at date in ISO Date Format then use $gte and $lte.
db.connection.find({
created_at: {
$gte: ISODate("2010-05-30T18:47:00.000Z"),
$lte: ISODate("2010-05-30T19:00:00.000Z")
}
})

use $gte and $lte to find between date data's in mongodb
var tomorrowDate = moment(new Date()).add(1, 'days').format("YYYY-MM-DD");
db.collection.find({"plannedDeliveryDate":{ $gte: new Date(tomorrowDate +"T00:00:00.000Z"),$lte: new Date(tomorrowDate + "T23:59:59.999Z")}})

mongoose.model('ModelName').aggregate([
{
$match: {
userId: mongoose.Types.ObjectId(userId)
}
},
{
$project: {
dataList: {
$filter: {
input: "$dataList",
as: "item",
cond: {
$and: [
{
$gte: [ "$$item.dateTime", new Date(`2017-01-01T00:00:00.000Z`) ]
},
{
$lte: [ "$$item.dateTime", new Date(`2019-12-01T00:00:00.000Z`) ]
},
]
}
}
}
}
}
])

For those using Make (formerly Integromat) and MongoDB:
I was struggling to find the right way to query all records between two dates. In the end, all I had to do was to remove ISODate as suggested in some of the solutions here.
So the full code would be:
"created": {
"$gte": "2016-01-01T00:00:00.000Z",
"$lt": "2017-01-01T00:00:00.000Z"
}
This article helped me achieve my goal.
UPDATE
Another way to achieve the above code in Make (formerly Integromat) would be to use the parseDate function. So the code below will return the same result as the one above :
"created": {
"$gte": "{{parseDate("2016-01-01"; "YYYY-MM-DD")}}",
"$lt": "{{parseDate("2017-01-01"; "YYYY-MM-DD")}}"
}
⚠️ Be sure to wrap {{parseDate("2017-01-01"; "YYYY-MM-DD")}} between quotation marks.

Convert your dates to GMT timezone as you're stuffing them into Mongo. That way there's never a timezone issue. Then just do the math on the twitter/timezone field when you pull the data back out for presentation.

Why not convert the string to an integer of the form YYYYMMDDHHMMSS? Each increment of time would then create a larger integer, and you can filter on the integers instead of worrying about converting to ISO time.

Scala:
With joda DateTime and BSON syntax (reactivemongo):
val queryDateRangeForOneField = (start: DateTime, end: DateTime) =>
BSONDocument(
"created_at" -> BSONDocument(
"$gte" -> BSONDateTime(start.millisOfDay().withMinimumValue().getMillis),
"$lte" -> BSONDateTime(end.millisOfDay().withMaximumValue().getMillis)),
)
where millisOfDay().withMinimumValue() for "2021-09-08T06:42:51.697Z" will be "2021-09-08T00:00:00.000Z"
and
where millisOfDay(). withMaximumValue() for "2021-09-08T06:42:51.697Z" will be "2021-09-08T23:59:99.999Z"

i tried in this model as per my requirements i need to store a date when ever a object is created later i want to retrieve all the records (documents ) between two dates
in my html file
i was using the following format mm/dd/yyyy
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<script>
//jquery
$(document).ready(function(){
$("#select_date").click(function() {
$.ajax({
type: "post",
url: "xxx",
datatype: "html",
data: $("#period").serialize(),
success: function(data){
alert(data);
} ,//success
}); //event triggered
});//ajax
});//jquery
</script>
<title></title>
</head>
<body>
<form id="period" name='period'>
from <input id="selecteddate" name="selecteddate1" type="text"> to
<input id="select_date" type="button" value="selected">
</form>
</body>
</html>
in my py (python) file i converted it into "iso fomate"
in following way
date_str1 = request.POST["SelectedDate1"]
SelectedDate1 = datetime.datetime.strptime(date_str1, '%m/%d/%Y').isoformat()
and saved in my dbmongo collection with "SelectedDate" as field in my collection
to retrieve data or documents between to 2 dates i used following query
db.collection.find( "SelectedDate": {'$gte': SelectedDate1,'$lt': SelectedDate2}})

How do I use Field Selection to return a contained field with the Mongo shell?

I am storing Tweets in a MongoDB database, and would like to use Field Selectors (http://www.mongodb.org/display/DOCS/Querying#Querying-FieldSelection) to return just the text, the date and the screen name of the user.
{
"_id" : NumberLong("213686009408"),
"text" : "RT #Milanello: Lightning over the Donb...",
"created_at" : "Fri, 15 Jun 2012 17:37:38 +0000",
"user" : {
"screen_name" : "xxxxx",
"profile_image_url" : "http://a0.twimg.com/pro[...].jpg"
}
}
Creating a query that returns just the text and date is easy enough:
> db.abc.find({},{created_at:1,text:1})
I'm not sure how to return the user.screen_name property as well?
[UPDATE]
I had previously tried using the dot notation and it threw an exception, however, as per both answers below (I have +1'd both) the dot notation does work, but only if you use "" around the attribute names.
So, this does NOT work:
db.abc.find({},{created_at:1,text:1, user.screen_name:1})
But this DOES:
db.abc.find({},{created_at:1,text:1, "user.screen_name":1})

You can use dot notation as you would in a query:
db.abc.find({},{created_at:1,text:1, "user.screen_name" : 1})
However, the field will not be flattened, so the result objects still have a user-subdocument:
{ "_id" : NumberLong("4546545"),
"created_at" : "Fri, 15 Jun...",
"user" : { "screen_name" : "John Doe" }
}
However, flattening this in code shouldn't be a big problem.

You need to use dot notation to query for values within the array. I created a similar document, modified the _id field, and ran the following queries successfully -
> db.abc.find({},{'user.screen_name': 1})
{ "_id" : 1200, "user" : { "screen_name" : "xxxxx" } }
> db.abc.find({'user.screen_name': "xxxxx"})
{ "_id" : 1200, "text" : "RT #Milanello: Lightning over the Donb...", "created_at" : "Fri, 15 Jun 2012 17:37:38 +0000", "user" : { "screen_name" : "xxxxx", "profile_image_url" : "http://a0.twimg.com/pro[...].jpg" } }
There's more infomation on advanced queries here.
Update: Looks like I posted this 2 minutes after mnemosyn so accept his answer first.