The and() criterion behaves incorrectly when Ruby symbols are passed instead of strings. The repro below demonstrates this issue. This issue is fixed by converting potential Ruby symbol usages to strings in the and() code.

Repro

For some DB, "Employees":

Band.create!
Band.create!
e_ids = Band.all.take(2).map(&:id); nil

=> [BSON::ObjectId('<oid_1>'), BSON::ObjectId('<oid_2>')]

Band.where(:id => { "$in" => e_ids }).selector

=> {"_id"=>{"$in"=>[BSON::ObjectId('<oid_1>')]}}

# specifying both as strings "$in" works -

Band.where(:id => { "$in" => e_ids }).and(:id => { "$in" => [ e_ids[1] ] }).selector
=> {"_id"=>{"$in"=>[BSON::ObjectId('<oid_1>')]}, "$and"=>[{"_id"=>{"$in"=>[BSON::ObjectId('<oid_1>')]}}]}

Employees.where(:id => { "$in" => e_ids }).and(:id => { "$in" => [ e_ids[1] ] }).count

=> 1

# specifying one as a symbol does not work -

Band.where(:id => { "$in" => e_ids }).and(:id => { :$in => [ e_ids[1] ] }).selector

NoMethodError: undefined method `start_with?' for :$in:Symbol

# specifying both as symbols also does not work

Band.where(:id => {:$in => e_ids }).and(:id => { :$in => [ e_ids[1] ] }).selector

NoMethodError: undefined method `start_with?' for :$in:Symbol

# but specifying one as a symbol without the and() does work

Band.where(:id => { :$in => e_ids }).selector

=> {"_id"=>{:$in=>[BSON::ObjectId('<oid_1>')]}}

Band.where(:id => { :$in => e_ids }).count
=> 2