Validating Mongo collections
Well, firstly: "string".to_s is entirely redundant. That's already a String, so converting it again is just wasting your (processor's) time. 

Secondly, writing if blocks with just one line and no else wastes a good deal of vertical real estate. I'd recommend that instead of 

if condition
  statement
end


you should write

statement if condition


Ruby has unless as the opposite to if, and when you find yourself writing if !..., it's better to use unless ... instead. This is mostly because of convention -- Ruby programmers use unless a lot more, so it's quicker to parse.

As for shortening the method calls themselves, you could just pass strict: false in with the name, so each individual call becomes more like this:

db.create_collection('name', strict: false)


Then you don't need the condition at all.

Then, since you have three objects on which you want to perform the same action, you can put them in an array:

['reports', 'days', 'users']


(or use the fancier %w{ reports days users }, like I do below, but you'll want to read up on its gotchas first)

Then we can loop through that array:

%w{ reports days users }.each do |name|
  #...
end


And in place of #... we put the code to create the table:

db.create_collection(name, strict: false)


Or, if you don't want to rely on swallowing errors:

db.create_collection(name) unless db.collection_names.include?(name)


If we put that all together, it becomes this:

%w{ reports days users }.each do |name|
  db.create_collection(name) unless db.collection_names.include?(name)
end


Now, to add a collection, all you have to do is add it to the list. For example, if you wanted to add one called "years", add years in to the %w:

%w{ reports days users years }.each do |name|
  db.create_collection(name) unless db.collection_names.include?(name)
end