mongodb - exception: can't convert from BSON type EOO to Date

Question

I am getting an issue for running the following aggregate query:

db.snippets.aggregate([ { '$project': { month: { '$month': '$created_at' }} } ])

The error message for the same is:

assert: command failed: {
        "errmsg" : "exception: can't convert from BSON type EOO to Date",
        "code" : 16006,
        "ok" : 0 } : aggregate failed

How do I get around this issue? I found a related question: MongoDB: can't convert from BSON type EOO to Date.

But it doesn't tell how to get things done.

Answer 1

You likely have one or more docs with a created_at value that's not a BSON Date and you'll need to fix that by converting those values to Date or removing them.

You can find those docs with a $not query that uses the $type operator like:

db.snippets.find({created_at: {$not: {$type: 9}}})

If the created_at values are date strings, you can find the docs that need updating and then update them in the shell using code like:

db.snippets.find({created_at: {$not: {$type: 9}}}).forEach(function(doc) {
    // Convert created_at to a Date 
    doc.created_at = new Date(doc.created_at);
    db.snippets.save(doc);
})