Hi
In the mongodb docs it states:
When rounding on a value of
5,$roundrounds to the nearest even value.
https://www.mongodb.com/docs/manual/reference/operator/aggregation/round/#rounding-to-even-values
However, when checking the following aggregation example:
[{$project: {
'1905': {$round: [1905,-1]},
'1915': {$round: [1915,-1]},
'1925': {$round: [1925,-1]},
'1935': {$round: [1935,-1]},
'1945': {$round: [1945,-1]},
'1955': {$round: [1955,-1]},
'1965': {$round: [1965,-1]},
'1975': {$round: [1975,-1]},
'1985': {$round: [1985,-1]},
'1995': {$round: [1995,-1]},
'190,5': {$round: [190.5,0]},
'191,5': {$round: [191.5,0]},
'192,5': {$round: [192.5,0]},
'193,5': {$round: [193.5,0]},
'194,5': {$round: [194.5,0]},
'195,5': {$round: [195.5,0]},
'196,5': {$round: [196.5,0]},
'197,5': {$round: [197.5,0]},
'198,5': {$round: [198.5,0]},
'199,5': {$round: [199.5,0]},
'19,05': {$round: [19.05,1]},
'19,15': {$round: [19.15,1]},
'19,25': {$round: [19.25,1]},
'19,35': {$round: [19.35,1]},
'19,45': {$round: [19.45,1]},
'19,55': {$round: [19.55,1]},
'19,65': {$round: [19.65,1]},
'19,75': {$round: [19.75,1]},
'19,85': {$round: [19.85,1]},
'19,95': {$round: [19.95,1]},
'19,005': {$round: [19.005,2]},
'19,015': {$round: [19.015,2]},
'19,025': {$round: [19.025,2]},
'19,035': {$round: [19.035,2]},
'19,045': {$round: [19.045,2]},
'19,055': {$round: [19.055,2]},
'19,065': {$round: [19.065,2]},
'19,075': {$round: [19.075,2]},
'19,085': {$round: [19.085,2]},
'19,095': {$round: [19.095,2]},
'19,0005': {$round: [19.0005,3]},
'19,0015': {$round: [19.0015,3]},
'19,0025': {$round: [19.0025,3]},
'19,0035': {$round: [19.0035,3]},
'19,0045': {$round: [19.0045,3]},
'19,0055': {$round: [19.0055,3]},
'19,0065': {$round: [19.0065,3]},
'19,0075': {$round: [19.0075,3]},
'19,0085': {$round: [19.0085,3]},
'19,0095': {$round: [19.0095,3]},
'19,00005': {$round: [19.00005,4]},
'19,00015': {$round: [19.00015,4]},
'19,00025': {$round: [19.00025,4]},
'19,00035': {$round: [19.00035,4]},
'19,00045': {$round: [19.00045,4]},
'19,00055': {$round: [19.00055,4]},
'19,00065': {$round: [19.00065,4]},
'19,00075': {$round: [19.00075,4]},
'19,00085': {$round: [19.00085,4]},
'19,00095': {$round: [19.00095,4]},
'19,000005': {$round: [19.000005,5]},
'19,000015': {$round: [19.000015,5]},
'19,000025': {$round: [19.000025,5]},
'19,000035': {$round: [19.000035,5]},
'19,000045': {$round: [19.000045,5]},
'19,000055': {$round: [19.000055,5]},
'19,000065': {$round: [19.000065,5]},
'19,000075': {$round: [19.000075,5]},
'19,000085': {$round: [19.000085,5]},
'19,000095': {$round: [19.000095,5]},
}}]
The following results are returned:
"1905" : 1900.0,
"1915" : 1920.0,
"1925" : 1920.0,
"1935" : 1940.0,
"1945" : 1940.0,
"1955" : 1960.0,
"1965" : 1960.0,
"1975" : 1980.0,
"1985" : 1980.0,
"1995" : 2000.0,
"190,5" : 190.0,
"191,5" : 192.0,
"192,5" : 192.0,
"193,5" : 194.0,
"194,5" : 194.0,
"195,5" : 196.0,
"196,5" : 196.0,
"197,5" : 198.0,
"198,5" : 198.0,
"199,5" : 200.0,
"19,05" : 19.1,
"19,15" : 19.1,
"19,25" : 19.2,
"19,35" : 19.4,
"19,45" : 19.4,
"19,55" : 19.6,
"19,65" : 19.6,
"19,75" : 19.8,
"19,85" : 19.9,
"19,95" : 19.9,
"19,005" : 19.0,
"19,015" : 19.02,
"19,025" : 19.02,
"19,035" : 19.04,
"19,045" : 19.05,
"19,055" : 19.05,
"19,065" : 19.07,
"19,075" : 19.07,
"19,085" : 19.09,
"19,095" : 19.09,
"19,0005" : 19.0,
"19,0015" : 19.002,
"19,0025" : 19.003,
"19,0035" : 19.003,
"19,0045" : 19.005,
"19,0055" : 19.006,
"19,0065" : 19.006,
"19,0075" : 19.008,
"19,0085" : 19.009,
"19,0095" : 19.009,
"19,00005" : 19.0001,
"19,00015" : 19.0002,
"19,00025" : 19.0003,
"19,00035" : 19.0004,
"19,00045" : 19.0005,
"19,00055" : 19.0006,
"19,00065" : 19.0007,
"19,00075" : 19.0008,
"19,00085" : 19.0008,
"19,00095" : 19.0009,
"19,000005" : 19.00001,
"19,000015" : 19.00002,
"19,000025" : 19.00003,
"19,000035" : 19.00004,
"19,000045" : 19.00005,
"19,000055" : 19.00005,
"19,000065" : 19.00006,
"19,000075" : 19.00007,
"19,000085" : 19.00008,
"19,000095" : 19.0001
As far as I can tell it does not match the described behaviour in the docs when rounding decimal values.
Am I missing something or is there a bug in the behaviour for $round?