I love MongoDB, and a certain little ambiguity occurred to me and I was wondering if anyone had seen this before and possibly would know the answer :-).
in mongo, to reach into sub-objects, you use dot notation, for example:
db.persons.find({ "address.state" : "CA" })
which is simple enough. How (if it does at all) does mongo deal with the difference between:
{
"address" { "state" : "CA" }
}
and
{
"address.state" : "CA"
}
since dots are legal in keys as far as i know. Additionally, I believe that this would be a legal doc as well:
{
"address" { "state" : "A" },
"address.state" : "B"
}
in which case, I can see this query returning either "A" or "B":
db.persons.find({}, {"address.state"}) // all docs selecting address.state as result.
Similar potential issue can arise I imagine with arrays as well:
{"a":["test"]}
which could be access with:
{"a.0"}
and of course
{"a" {"0" : "test"} }
which would also be access with:
{"a.0"}
thoughts? experiences? Is the conventional wisdom simply not to do that?
A key such as "address.state" isn't legal. From here:
Field names cannot contain dots (i.e. .) or null characters, and they must not start with a dollar sign (i.e. $).
Related
Before I explain my use case, I'd like to state that yes, I could change this application so that it would store things in a different manner or even split it into 2 collections for that matter. But that's not my intention, and I'd rather want to know if this is at all possible within MongoDB (since I am quite new to MongoDB). I can for sure work around this problem if I'd really need to, but rather looking for a method to achieve what I want (no I am not being lazy here, I really want to know a way to do this).
Let's get to the problem then.
I have a document like below:
{
"_id" : ObjectId("XXXXXXXXXXXXXXXXXXXXX"),
"userId" : "XXXXXXX",
"licenses" : [
{
"domain" : "domain1.com",
"addons" : [
{"slug" : "1"},
{"slug" : "2"}
]
},
{
"domain" : "domain2.com",
"addons" : [
{"slug" : "1"},
]
}
]
}
My goal is to check if a specific domain has a specific addon. When I use the below query to count the documents with domain: domain2.com and addon slug: 2 the result should be: 0. However with the below query it returns 1. I know that this is because the query is executed document wide and not just the license index that matched domain2.com. So my question is, how to do a sub $and (or however you'd call it)?
db.test.countDocuments(
{$and: [
{"licenses.domain": "domain2.com"},
{"licenses.addons.slug": "2"},
]}
)
Basically I am looking for something like this (below isn't working obviously), but below should return 0, not 1:
db.test.countDocuments(
{$and: [
{
"licenses.domain": "domain2.com",
$and: [
{ "licenses.addons.slug": "2"}
]
}
]}
)
I know there is $group and $filter operators, I have been trying many combinations to no avail. I am lost at this point, I feel like I am completely missing the logic of Mongo here. However I believe this must be relatively easy to accomplish with a single query (just not for me I guess).
I have been trying to find my answer on the official documentation and via stack overflow/google, but I really couldn't find any such use case.
Any help is greatly appreciated! Thanks :)
What you are describe is searching for a document whose array contains a single element that matches multiple criteria.
This is exactly what the $elemMatch operator does.
Try using this for the filter part:
{
licenses: {
$elemMatch: {
domain: "domain2.com",
"addons.slug": "2"
}
}
}
I got the unusual json (actually from IBM Bluemix), shown below,
Thank goodness, trusty and heartwarming SwiftyJSON was able to get the values, like this...
let mauves = json["blue"][0]["brown"][0]["mauve"]
However, notice there are weird sort of "empty unnamed array nested things" in the JSON (hence the [0] calls to Swifty).
My question, in short,
is this valid json?
Even if valid, is it "crappy"? Or am I wrong, it's totally idiomatic? (Maybe I've just been dating the wrong services for decades, I don't know.)
I appreciate that running it through online validators seems to say "valid" (except this one http://json.parser.online.fr gives red things), but, you know, who trusts online services? Ask experts on SO....)
--
{
"red" : 1,
"green" : 4,
"blue" : [
{
"yellow" : "word",
"brown" : [
{
"orange" : "1826662593",
"gold" : "23123",
"mauve" : [
{
"a" : "Beagle",
"b" : 0.979831
},
{
"a" : "Chow",
"b" : 0.937588
},
{
"a" : "Hound",
"b" : 0.987798
}
]
}
]
}
]
}
--
The JSON is valid. The blue member contains an array with 1 element (at index [0] which is the yellow object, and this is repeated for orange.
When I paste it into json.parser.online.fr it reports it as valid for me - are you accidentally including other text around it?
The JSON is perfectly valid - your validators are not lying to you. I don't know if this JSON contains real keys, or if the names have been changed to protect the innocent (it certainly looks like nonsense), but in a real world situation there are frequently arrays that contain one element (because they might contain zero or many elements!).
This is the case: A webshop in which I want to configure which items should be listed in the sjop based on a set of parameters.
I want this to be configurable, because that allows me to experiment with different parameters also change their values easily.
I have a Product collection that I want to query based on multiple parameters.
A couple of these are found here:
within product:
"delivery" : {
"maximum_delivery_days" : 30,
"average_delivery_days" : 10,
"source" : 1,
"filling_rate" : 85,
"stock" : 0
}
but also other parameters exist.
An example of such query to decide whether or not to include a product could be:
"$or" : [
{
"delivery.stock" : 1
},
{
"$or" : [
{
"$and" : [
{
"delivery.maximum_delivery_days" : {
"$lt" : 60
}
},
{
"delivery.filling_rate" : {
"$gt" : 90
}
}
]
},
{
"$and" : [
{
"delivery.maximum_delivery_days" : {
"$lt" : 40
}
},
{
"delivery.filling_rate" : {
"$gt" : 80
}
}
]
},
{
"$and" : [
{
"delivery.delivery_days" : {
"$lt" : 25
}
},
{
"delivery.filling_rate" : {
"$gt" : 70
}
}
]
}
]
}
]
Now to make this configurable, I need to be able to handle boolean logic, parameters and values.
So, I got the idea, since such query itself is JSON, to store it in Mongo and have my Java app retrieve it.
Next thing is using it in the filter (e.g. find, or whatever) and work on the corresponding selection of products.
The advantage of this approach is that I can actually analyse the data and the effectiveness of the query outside of my program.
I would store it by name in the database. E.g.
{
"name": "query1",
"query": { the thing printed above starting with "$or"... }
}
using:
db.queries.insert({
"name" : "query1",
"query": { the thing printed above starting with "$or"... }
})
Which results in:
2016-03-27T14:43:37.265+0200 E QUERY Error: field names cannot start with $ [$or]
at Error (<anonymous>)
at DBCollection._validateForStorage (src/mongo/shell/collection.js:161:19)
at DBCollection._validateForStorage (src/mongo/shell/collection.js:165:18)
at insert (src/mongo/shell/bulk_api.js:646:20)
at DBCollection.insert (src/mongo/shell/collection.js:243:18)
at (shell):1:12 at src/mongo/shell/collection.js:161
But I CAN STORE it using Robomongo, but not always. Obviously I am doing something wrong. But I have NO IDEA what it is.
If it fails, and I create a brand new collection and try again, it succeeds. Weird stuff that goes beyond what I can comprehend.
But when I try updating values in the "query", changes are not going through. Never. Not even sometimes.
I can however create a new object and discard the previous one. So, the workaround is there.
db.queries.update(
{"name": "query1"},
{"$set": {
... update goes here ...
}
}
)
doing this results in:
WriteResult({
"nMatched" : 0,
"nUpserted" : 0,
"nModified" : 0,
"writeError" : {
"code" : 52,
"errmsg" : "The dollar ($) prefixed field '$or' in 'action.$or' is not valid for storage."
}
})
seems pretty close to the other message above.
Needles to say, I am pretty clueless about what is going on here, so I hope some of the wizzards here are able to shed some light on the matter
I think the error message contains the important info you need to consider:
QUERY Error: field names cannot start with $
Since you are trying to store a query (or part of one) in a document, you'll end up with attribute names that contain mongo operator keywords (such as $or, $ne, $gt). The mongo documentation actually references this exact scenario - emphasis added
Field names cannot contain dots (i.e. .) or null characters, and they must not start with a dollar sign (i.e. $)...
I wouldn't trust 3rd party applications such as Robomongo in these instances. I suggest debugging/testing this issue directly in the mongo shell.
My suggestion would be to store an escaped version of the query in your document as to not interfere with reserved operator keywords. You can use the available JSON.stringify(my_obj); to encode your partial query into a string and then parse/decode it when you choose to retrieve it later on: JSON.parse(escaped_query_string_from_db)
Your approach of storing the query as a JSON object in MongoDB is not viable.
You could potentially store your query logic and fields in MongoDB, but you have to have an external app build the query with the proper MongoDB syntax.
MongoDB queries contain operators, and some of those have special characters in them.
There are rules for mongoDB filed names. These rules do not allow for special characters.
Look here: https://docs.mongodb.org/manual/reference/limits/#Restrictions-on-Field-Names
The probable reason you can sometimes successfully create the doc using Robomongo is because Robomongo is transforming your query into a string and properly escaping the special characters as it sends it to MongoDB.
This also explains why your attempt to update them never works. You tried to create a document, but instead created something that is a string object, so your update conditions are probably not retrieving any docs.
I see two problems with your approach.
In following query
db.queries.insert({
"name" : "query1",
"query": { the thing printed above starting with "$or"... }
})
a valid JSON expects key, value pair. here in "query" you are storing an object without a key. You have two options. either store query as text or create another key inside curly braces.
Second problem is, you are storing query values without wrapping in quotes. All string values must be wrapped in quotes.
so your final document should appear as
db.queries.insert({
"name" : "query1",
"query": 'the thing printed above starting with "$or"... '
})
Now try, it should work.
Obviously my attempt to store a query in mongo the way I did was foolish as became clear from the answers from both #bigdatakid and #lix. So what I finally did was this: I altered the naming of the fields to comply to the mongo requirements.
E.g. instead of $or I used _$or etc. and instead of using a . inside the name I used a #. Both of which I am replacing in my Java code.
This way I can still easily try and test the queries outside of my program. In my Java program I just change the names and use the query. Using just 2 lines of code. It simply works now. Thanks guys for the suggestions you made.
String documentAsString = query.toJson().replaceAll("_\\$", "\\$").replaceAll("#", ".");
Object q = JSON.parse(documentAsString);
Can someone please tell me the difference b/w following two queries. Both works for me and seems to be giving correct results, but i am not sure if really there is any difference or not
Retrieve all students having scores between 80 and 95
var query1 = { 'grade' : {"$gt":80}, 'grade' : {"$lt":95} };
var query2 = { 'grade' : {"$gt":80,"$lt":95} };
I think that you will find that your first form does not actually work, even if it did for you on a minimal sample. And it will fail for a very good reason. Consider the following documents:
{ "grade" : 90 }
{ "grade" : 96 }
{ "grade" : 80 }
If you issue your first query form you will get this for a result:
{ "grade" : 90 }
{ "grade" : 80 }
The reason being that you cannot have the "same key" in a document like this, and one will negate the other. In this case the "right" side key takes precedence over the left key and overwrites. This is common behavior for hash or dictionary structures.
This is why the second form is required and will of course return only the document that matches the conditions that are intended to be specified.
When you have an actual case to use the same field and probably using different conditions you can use the $and operator. Not the best example, but just to clarify:
db.collection.find({ "$and": [
{ "grade": { "$gt": 80 } },
{ "grade": { "$lt": 95 } },
]})
It's real purpose is to combine different conditions on the same field.
For your case though, use the second form you specified.
You should use the second query for the following reason: you query is actually a JSON. And in your first query you are providing a duplicate key (grade).
Nonetheless they are permitted by the JSON RFC, however even Doug Crockford mentioned that he regrets leaving this ambiguity in the spec because it inevitably leads to all kinds of confusion.
Maybe Mongo parses it correctly right now (or may be rather in your case), but you do not know for how long will it be this way (and some other json parsers tells you that you have an error).
So the best way is to think about the first query as bad and only use second one.
I would like to construct a query that will return just the names of the classes for the datastructure below.
So far, the closest I've come is using dot notation
db.mycoll.find({name:"game1"},{"classes.1.name":true})
But the problem with this approach is that it will only return the name of the first class. Please help me get the names of all three classes.
I wish I could use a wild card as below, but I'm not sure if that exists.
db.mycoll.find({name:"game1"},{"classes.$*.name":true})
Datastructure:
{
"name" : "game1",
"classes" : {
"1" : {
"name" : "warlock",
"version" : "1.0"
},
"2" : {
"name" : "shaman",
"version" : "2.0"
},
"3" : {
"name" : "mage",
"version" : "1.0"
}
}
There is no simple query that will achieve the results you seek. MongoDB has limited support for querying against sub-objects or arrays of objects. The basic premise with MongoDB is that you are querying for the top-level document.
That said, things are changing and you still have some options:
Use the new MongoDB Aggregation Framework. This has a $project operation that should do what you're looking for.
You can return just the classes field and then merge the names together. This should be trivial with most languages.
Note, that it's not clear what you're doing with classes. Is this an array or an object? If it's an object, what does classes.1 actually represent? Is it different from classes.warlock?