Why the indexonly attribute value is false from mongodb explain query output - mongodb

Question

I have a simple collection named customers as shown below
db.customers.find().pretty()
{
"_id" : ObjectId("524eb09ca71b72672e65ebb6"),
"name" : "kiran",
"occupation" : "SelfEmployeed",
"country" : "IND"
}
{
"_id" : ObjectId("524eb0a4a71b72672e65ebb7"),
"name" : "Mark",
"occupation" : "Architect",
"country" : "US"
}
{
"_id" : ObjectId("524eb0aba71b72672e65ebb8"),
"name" : "beast",
"occupation" : "housewife",
"country" : "UK"
}
{
"_id" : ObjectId("524eb0b2a71b72672e65ebb9"),
"name" : "Philip",
"occupation" : "Engineer",
"country" : "SWE"
}
I have created indexes on the name and country fields as shown below
db.customers.ensureIndex({name : 1}, {"unique" : false})
db.customers.ensureIndex({country : 1}, {"unique" : false})
The indexes have been created
db.customers.getIndexKeys()
[ { "_id" : 1 }, { "name" : 1 }, { "country" : 1 } ]
This was the result of my query explain
db.customers.find({name : "Mark"}).explain()
{
"cursor" : "BtreeCursor name_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"name" : [
[
"Mark",
"Mark"
]
]
},
"server" : "************"
}
Why the indexonly attribute value is false ??
I have seen similar question which explains it is due to
indexonly is false as It wont use index only because you'll be retrieving other fields via that query that aren't indexed.
Please let me know what does other fields mean here ??

Answer 1

Some searching would have actually have got you a question I answered some time ago on this very subject.
The reason is this:
db.customers.find({name : "Mark"}).explain()
There is no projection, how can MongoDB know that the index covers the return without looking at the actual documents in this case?
It is similar to
SELECT * from customers
And
SELECT d from customers
How can you know * is d without looking?