MongoDB: update every document on one field

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)