我会这样建模：

class Tree
  has_many :tree_locations
  has_many :countries, through: :tree_locations
  has_many :native_tree_locations, class_name: 'TreeLocation', 
                              -> { where(native: true) }
  has_many :native_countries, through: :native_tree_locations, 
                              class_name: 'country'

class TreeLocation
  belongs_to :tree
  belongs_to :country
  # and a `native` boolean flag in the DB

class Country
  has_many :tree_locations
  has_many :trees, through: :tree_locations
  # ...

这允许给定的tree：

• tree.countries将返回树生长的所有国家/地区，但是
• tree.native_countries将仅返回该树所在的国家/地区。