Question

I have a collected named foo hypothetically.

Each instance of foo has a field called lastLookedAt which is a UNIX timestamp since epoch. I’d like to be able to go through the MongoDB client and set that timestamp for all existing documents (about 20,000 of them) to the current timestamp.

What’s the best way of handling this?

Answer 1

Regardless of the version, for your example, the <update> is:

{  $set: { lastLookedAt: Date.now() / 1000 }  }

However, depending on your version of MongoDB, the query will look different. Regardless of version, the key is that the empty condition {} will match any document. In the Mongo shell, or with any MongoDB client:

$version >= 3.2:

db.foo.updateMany( {}, <update> )

• {} is the condition (the empty condition matches any document)

3.2 > $version >= 2.2:

db.foo.update( {}, <update>, { multi: true } )

• {} is the condition (the empty condition matches any document)
• {multi: true} is the “update multiple documents” option

$version < 2.2:

db.foo.update( {}, <update>, false, true )

• {} is the condition (the empty condition matches any document)
• false is for the “upsert” parameter
• true is for the “multi” parameter (update multiple records)