If you have a collection, "test" containing 3 documents.

test = db.collection('test')
test.insert(

{ 'a' => 1 }

)
test.insert(

{ 'b' => 1 }

)
test.insert(

{ 'c' => 1 }

)

cursor = test.find
puts cursor.count

1. => 3

puts cursor.first

1. => { '_id' => ..., 'a' => 1}

cursor.each do |doc|
puts doc
end

1. => { '_id' => ..., 'b' => 1 }
2. => { '_id' => ..., 'c' => 1 }

I would expect the call to #first to be non-destructive so that a later iteration returns all 3 documents.

A possible to workaround is to use each_with_index and checking if the index == 0