Capped collections documentation at https://www.mongodb.com/docs/manual/core/capped-collections/ specifies that

"TTL indexes are not compatible with capped collections."

However, the following tests with an empty database (i.e. no collections in the database when the test is done):

{{> show collections
> db.createCollection( "c",

{ capped: true, size: 100000 }

)

{ "ok" : 1 }

> db.c.isCapped()
true
> db.c.createIndex(

{ "date": 1 }

,

{ expireAfterSeconds: 3600 }

)

{ "numIndexesBefore" : 1, "numIndexesAfter" : 2, "createdCollectionAutomatically" : false, "ok" : 1 }

> db.c.getIndexes()
[
{
"v" : 2,
"key" :

{ "_id" : 1 }

,
"name" : "id"
},
{
"v" : 2,
"key" :

{ "date" : 1 }

,
"name" : "date_1",
"expireAfterSeconds" : 3600
}
]}}

So, I understands the documentation is not correct or "compatibility" should be explained better (i.e. the index can be created but expiration on it is ignored).

 

---

With MongoDB 7.0.0-rc4 and MongoDB 6.0.7

Enterprise replset [direct: primary] test> db.createCollection( "c", {capped:true, size:100000})
{ ok: 1 }
Enterprise replset [direct: primary] test> db.c.isCapped()
true
Enterprise replset [direct: primary] test> db.c.createIndex({"date":1},{expireAfterSeconds:3600})
MongoServerError: Cannot create TTL index on a capped collection

With MongoDB 5.0.18:

test> db.createCollection( "c", {capped:true, size:100000})
{ ok: 1 }
test> db.c.isCapped()
true
test> db.c.createIndex({"date":1},{expireAfterSeconds:3600})
date_1

Will update the 5.0 docs and backport to 4.4 and 4.2