Shorten mapping and concat

I get an array with person objects. A person can have several houses:

 person.houses => ActiveRecord_Associations_CollectionProxy

Some houses can belong to several persons. In my code I would like to get all unique houses for the persons that are in the Array.

This is a long version of the code:

 def persons_houses(persons)
   unique_houses = []

   persons.each do |person|
     person.houses.each do |house|
        unique_houses << house if !unique_houses.include? house
     end
   end

   unique_houses
 end

Do you know a shorter code for this persons_houses(persons) method?

I was thinking of mapping the persons houses, concat them at the same time and then return unique values

Something like: (But this is not valid Ruby)

persons.map { |person| concat(person.houses) }.uniq

Thanks for your help!