gistfile1.txt

# Using rails-3.0.0.beta4, mongoid-2.0.0.beta7, mongodb-1.4.4:

Article.where(:published_at.gte => Time.utc(2010,7,7)).to_a
# => [] 

Article.where(:published_at.lte => Time.utc(2010,7,7,23,59,59)).to_a
# => [#<Article _id: 4c2a75f5468ee3c930000009, created_at: 2010-06-29 22:38:45 UTC, updated_at: 2010-07-07 13:20:00 UTC, published_at: 2010-07-06 23:45:00 UTC, title: "Foo", slug: "foo", text: "Foo", tags: [], views: 0, site_id: BSON::ObjectID('4c2a73f0468ee3c930000006'), author_id: BSON::ObjectID('4c2a75d6468ee3ca6b000001')>] 

Article.where(:published_at.gte => Time.utc(2010,7,7)).where(:published_at.lte => Time.utc(2010,7,7,23,59,59)).to_a
# => [#<Article _id: 4c2a75f5468ee3c930000009, created_at: 2010-06-29 22:38:45 UTC, updated_at: 2010-07-07 13:20:00 UTC, published_at: 2010-07-06 23:45:00 UTC, title: "Foo", slug: "foo", text: "Foo", tags: [], views: 0, site_id: BSON::ObjectID('4c2a73f0468ee3c930000006'), author_id: BSON::ObjectID('4c2a75d6468ee3ca6b000001')>] 
# huh?

Seems like you cannot chain criterias on the same attribute?