mongodb - $dateFromString to support multiple format - mongodb

Question

I have a string field in mongodb which should be converted to a date field.
The format of the string is like the following:
2014 - Only year, default month and day are 01 and 01, so it should be converted to date '2014-01-01'
2014-01 - With year and month, which should also be converted to date '2014-01-01'
2014-01-01 - Full date
$dateFromString in the following syntax doesn't seem to work:
$dateFromString: {
dateString: '$order.date',
format: '%Y-%m-%d',
}
How can I make $dateFromString to support multiple format?

Answer 1

What you could do is to add a new field via $addFields and then for its value create few if conditions using the $cond pipeline operator matching each of your date lengths (via $strLenCP) and concatenating the remaining parts (via $concat). Then since all of your date fields will now match the format %Y-%m-%d it should work ... like this:
db.getCollection('<YourCol>').aggregate([{
$addFields: {
dateFixed: {
$cond: {
if: { $eq: [{ $strLenCP: "$date"}, 4] }, // <-- "2011"
then: { $concat: ["$date", "-01-01"] },
else: {
$cond: {
if: { $eq: [{ $strLenCP: "$date" }, 7] }, // <-- "2011-01"
then: { $concat: ["$date", "-01"] },
else: "$date" // <-- "2011-01-01"
}
}
}
}
}
},
{
$project: {
date: {
$dateFromString: {
dateString: 'dateFixed',
format: '%Y-%m-%d'
}
}
}
}
])
You can see it working here