Need to store high precision decimal values in MongoDB

Question

I have little experience with MongoDB. I am usual working on large scale SQL server DBs.

MongoDB only supports double and there is no decimal. The C# driver serializes decimals as strings.

What functionality do I miss if I store decimals as strings in MongoDB?
Is there a way to set a default serialization of decimals as double (AllowTruncation) without having to put an Attribute on each property?
What do I lose in precision if I used Bson double?

Thanks for your help!

UPDATE

I have an existing application model that uses decimals in C#. I want to use MongoDB as a new DB layer and change as little in the existing app as possible. Thats why I am looking for a way to map decimals in C# to double in MongoDB.
I understand that I loose precision and would have to analyze the side effects of it. My only remaining question is to know if there is a way to set a default serialization of decimals as double.

Thanks again. Great answers and comments so far.

If you need decimal precision, do not use doubles. You risk changing the data by doing so. — WiredPrairie, Nov 17 '13 at 23:21
possible duplicate of [MongoDB - What about Decimal type of value?](http://stackoverflow.com/questions/11541939/mongodb-what-about-decimal-type-of-value) — WiredPrairie, Nov 17 '13 at 23:21
.... and the still open issue: https://jira.mongodb.org/browse/SERVER-1393 — WiredPrairie, Nov 17 '13 at 23:22
Thanks for the responses! Great info.I still need to know how to set a default serialoization of decimals to double in C#. the question referred http://stackoverflow.com/questions/11541939/mongodb-what-about-decimal-type-of-value is not the same — PeterFromCologne, Nov 18 '13 at 13:05

score 8 · Accepted Answer · answered Nov 17 '13 at 22:47

I will answer your question partially (because I do not know much about C#).

So what will you lose if you will store decimals as strings.

your numbers on average would weight more (each double number cost 8 bytes to store which means that every string that has more then 8 chars will weight more). Because of these your indexes (if they will be built on this field would grow)
you will not be able to use operators which takes numbers as arguments $inc, $bit, $mod, $min, $max and in 2.6 version $mul. (May be I forgot something)
you will not be able to compare numbers (may be '1.65' and '1.23' is comparable as a string, but definitely not numbers with e and minuses somewhere in between). Because of this operations which build on top of comparison like $sort, and all these $gte, $gt, $lte, $lt will not work correctly.

What will you lose in precision if you store decimal as double:

based on this, Decimal in C# has 28-29 significant digits, whereas looking at my first link and checking the spec for double precision you see that it has 15-17 significant digits. This is basically what you will lose
another really important thing which people sometimes forget when dealing with double floats is illustrated below:

.

db.c.insert({_id : 1, b : 3.44}) 
db.c.update({_id : 1},{$inc : {b : 1}})
db.c.find({b: 4.44})  // WTf, where is my document? There is nothing there

Regarding the 2-nd subquestion:

Is there a way to set a default serialization of decimals as double (AllowTruncation) without having to put an Attribute on each property?

I do not really understood it, so I hope someone would be able to answer it.

Darxtar · Answer 2 · 2017-10-09T08:39:06.997

8

As of Mongodb 3.4 and the 2.4 Mongodb C# driver, decimal types are supported.

The properties of your document must have the [BsonRepresentation(BsonType.Decimal128)] attribute found in the MongoDB.Bson.Serialization.Attributes namespace.

this will map to "YourDecimalValue" : NumberDecimal("100.0000") in MongodDB. Robomongo supports the new decimal type from version 1.1 Beta.

edited Oct 09 '17 at 08:39

answered May 19 '17 at 11:34

Darxtar

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Many thanks for answering an almost 4 year old question! – PeterFromCologne Jun 10 '17 at 16:09

Need to store high precision decimal values in MongoDB

UPDATE

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