MongoDB exception: $concat only supports strings, not NUMBERINT32 solution _mongodb

Today, when I use MongoDB to manipulate aggregation, I want to format dates like "2013-10-17 04:41:37 UTC" into "October 17",

Copy Code code as follows:

' FDate ' => {' $concat ' => [' $date. Month ', ' month ', ' $date. Day ', ' th ']}

Appear exception: $concat only supports strings, not NumberInt32

Originally $concat can only manipulate strings, do not support numeric types, the solution is to use $SUBSTR

Copy Code code as follows:

$date-shaped like ' 2013-10-13 11:17:18 UTC '

' FDate ' => {' $concat ' => [{$substr =>[' $date ', 5, 2]}, ' month ', {$substr =>[' $date ', 8, 2]}, ' Day ']}

$SUBSTR accepts two parameters, one is the starting point of the string, and the other is the length of the intercepted string.