find(), by default, uses an AND operator to match multiple fields. If we want to use an OR operator, our query needs to be like this:

result = @collection.find('$or' => [{ isbn: '101' }, { isbn: '102' }]).to_a
puts result
{"_id"=>BSON::ObjectId('592149c4aabac953a3a1e31e'), "isbn"=>"101", "name"=>"Mastering MongoDB", "price"=>30.0, "published"=>2017-06-25 00:00:00 UTC}{"_id"=>BSON::ObjectId('59214bc1aabac954263b24e0'), "isbn"=>"102", "name"=>"MongoDB in 7 years", "price"=>50.0, "published"=>2017-06-26 00:00:00 UTC}

We can also use $and instead of $or in the previous example:

result = @collection.find('$and' => [{ isbn: '101' }, { isbn: '102' }]).to_a
puts result

This, of course, will return no results since no document can have both isbn 101 and 102.

An interesting and hard bug to find is if we define the same key multiple times, like this:

result = @collection.find({ isbn: '101', isbn: '102' })
puts result

{"_id"=>BSON::ObjectId('59214bc1aabac954263b24e0'), "isbn"=>"102", "name"=>"MongoDB in 7 years", "price"=>50.0, "published"=>2017-06-26 00:00:00 UTC}

Whereas the opposite order will cause the document with isbn 101 to be returned:

result = @collection.find({ isbn: '102', isbn: '101' })
puts result
{"_id"=>BSON::ObjectId('592149c4aabac953a3a1e31e'), "isbn"=>"101", "name"=>"Mastering MongoDB", "price"=>30.0, "published"=>2017-06-25 00:00:00 UTC}