Issues using $multiply with embedded documents

Discussion:

James Crosswell

2013-06-17 10:53:16 UTC

I can run this query against the aggregation framework:

{
$group :{
_id : {"_id" : "$exists"},
quantity: {$sum: "$Quantity"},
sales: {$sum: { $multiply : ["$Quantity", "$Quantity" ] }}
}
}

But not this one:

{
$group :{
_id : {"_id" : "$exists"},
quantity: {$sum: "$Quantity"},
sales: {$sum: { $multiply : ["$Quantity", "$UnitPrice.Amount" ] }}
}
}

The only difference is the second argument to the multiply function... in
the second query this is a field of an embedded document.

Is this a bug or do I have to use a special syntax to work with embedded
fields?

Cheers,
James
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Asya Kamsky

2013-06-17 15:12:58 UTC

Permalink

Both work fine for me. Could you show the exact error you are getting
(maybe cut-and-paste it along with the exact pipeline you are entering?)

Asya

Post by James Crosswell
{
$group :{
_id : {"_id" : "$exists"},
quantity: {$sum: "$Quantity"},
sales: {$sum: { $multiply : ["$Quantity", "$Quantity" ] }}
}
}
{
$group :{
_id : {"_id" : "$exists"},
quantity: {$sum: "$Quantity"},
sales: {$sum: { $multiply : ["$Quantity", "$UnitPrice.Amount" ] }}
}
}
The only difference is the second argument to the multiply function... in
the second query this is a field of an embedded document.
Is this a bug or do I have to use a special syntax to work with embedded
fields?
Cheers,
James

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See also the IRC channel -- freenode.net#mongodb

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James Crosswell

2013-06-17 20:23:47 UTC

Permalink

So it turns out the issue had nothing to do with the $multiply operation.
I'm getting at Mongo from c# and the UnitPrice.Amount field on the c# POCO
was a decimal (that's the recommended data type for dealing with currencies
in dotnet)... but Mongo can't handle decimal (128 bit) precision so it
encodes decimals as strings - which makes it impossible to perform
arithmetic operations on these. For the time being, I've worked around the
problem by storing UnitPrice.Amount as a double instead of a decimal. I
think that should still be enough precision for my use case... it's a bit
of a quirk of the c# driver perhaps though.

Cheers,
James

Post by Asya Kamsky
Both work fine for me. Could you show the exact error you are getting
(maybe cut-and-paste it along with the exact pipeline you are entering?)
Asya