question about $ in key names, like $schema and $ref

[Ognian]

Hello,

I'm just trying to put a json schema into mongodb.

In mongo $ and . are reserved chars and they are not allowed in key names

So I have a problem putting $schema or $ref inside the db.

Of course I can translate them when putting into the db an translate them back....

Don't kill me about this suggestion, but since it is a draft wouldn't it be possible to change $ref to _ref or __ref or something like this or at least add this possibility too...

Just an idea

Ognian

[Geraint]

Interesting problem...

I'm afraid "$ref" is not changing any time soon.  That's actually a separate standard (JSON Reference) which we simply use.  JSON Reference is quite mature now, and unlikely to budge.  Given that "$ref" is fixed (and the other problems it would cause), I think we're also unlikely to change "$schema".

I'm genuinely surprised that MongoDB has no way of escaping such characters.  MySQL has reserved characters as well (such as the backtick `, used for quoting names), but even these are escapable (using a double-backtick, e.g. Table`Name  ->  `Table``Name`).  Is there a way to raise this with MongoDB?

I had a look at the MongoDB site, and their FAQ suggested using a substitute value (http://docs.mongodb.org/manual/faq/developers/#faq-dollar-sign-escaping), so that's probably the way to go for now.

Sorry we can't be of more help!

[Geraint]

I've just skimmed through what MongoDB actually uses these $-names for, and one of the uses seems to be special representations of certain data types:

http://docs.mongodb.org/manual/reference/mongodb-extended-json/

For example, to encode a date, you would say: {"$date": <UTC epoch milliseconds>}

This actually seems fairly close to the use of "$schema", which identifies the particular format of the JSON Schema.  It's a shame they don't have JSON Schema support, actually - otherwise you could do queries like "get me this schema and all schemas it relies upon", or "get me any schema that includes this schema via using $ref", which otherwise require calculating and storing a whole bunch of meta-data yourself...

[Ognian]

OK, I've implemented the suggested workaround, (see below),

but it looks like this potentially could cause big misunderstanding since these two $ signs look pretty the same.... 

                        function mongoFixDollar(obj) {

                            // mongo forbids $ and . in keys so we use as recommended different unicode chars which look similar...

                            var ret = {};

                            for (var key in obj) {

                                ret[key.replace("＄", "$").replace("．", ".")] = obj[key];

                                if (typeof obj[key] === "object")

                                    mongoFixDollar(obj[key]);

                            }

                            return ret;

                        };

Ognian

[Geraint Luff]

Yeah, it's a bit unfortunate - you have to be sure that every section of code that gets things *out* of the database does the same substitution backwards.

I don't think it's just JSON Schema that's the problem - even if we did change those keywords, there are still plenty of other data formats that use JSON Reference, and you'd hit trouble with them as well.

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[Ognian]

Ups,

and the first implementation  of my "fixDollar" has a js beginner error; typeof is not what I tought ....

So here is the correct way to do it:

                        var toType = function (obj) {

                            return ({}).toString.call(obj).match(/\s([a-zA-Z]+)/)[1].toLowerCase();

                        };

                        var mongoFixDollar = function (obj) {

                            // mongo forbids $ and . in keys so we use as recommended different unicode chars which look similar...

                            var ret = {};

                            for (var key in obj) {

                                ret[key.replace("＄", "$").replace("．", ".")] = (toType(obj[key]) === "object") ? mongoFixDollar(obj[key]) : obj[key];

                            }

                            return ret;

                        };

magically this fixes my jsonary problem too...

[Austin Wright]

It's worth noting that these restrictions are only enforced at the library level. I'm perfectly capable of inserting keys with . and, I believe, $ in my database.

Nonetheless, I do escape my keys, using percent-encoding, but only for the three [$.%] characters, or, when a library calls for the $ character, I use the @ character instead (sometimes vice-versa too, since the $ character doesn't require quotes in ECMAScript, unlike @).

So there's multiple ways to deal with it.

But I agree it's rather silly it doesn't allow the full range of JSON.