ruby on rails - The bug of mongoid returning first document when invoking last? -

i encountered bug of mongoid, return first document when invoking method last

class post   include mongoid::document end  post.create post.create post.first == post.last #=> true 

the versions info:

• mongoid: "5.0.2"
• mongodb: v3.2.1
• rails 4.2.5

that's not bug in mongoid, that's bug in expectations of first , last methods. fine version 5 manual:

#first ⇒ document

note: mongoid added _id sort when no sort parameters provided explicitly user. caused bad performance issues , not expected, #first/#last no longer guarantee order if no sorting parameters provided. order guarantees - sort must explicitly provided.

get first document in database criteria's selector.

so first (as of mongoid5) gives first document in query with respect current order no longer supplies default order. you'll need supply own sort order in query if want first behave used to.

similarly last.