Implement 'import' action

app/controllers/default/units_controller.rb

@@ -1,7 +1,8 @@
 class Default::UnitsController < ApplicationController
   navigation_tab :units
 
-  before_action :find_unit, only: [:import, :export, :destroy]
+  before_action -> { find_unit(current_user) }, only: :export
+  before_action -> { find_unit(nil) }, only: [:import, :destroy]
 
   before_action except: :index do
     case action_name.to_sym
@@ -17,9 +18,16 @@ class Default::UnitsController < ApplicationController
   end
 
   def import
+    current_user.units
+      .find_or_initialize_by(symbol: @unit.symbol)
+      .update!(base: @base, **@unit.slice(:name, :multiplier))
+    run_and_render :index
   end
 
   def import_all
+    # From defaults_diff return not only portability, but reason for not being
+    # portable: missing_base and nesting_too_deep. Add portable and
+    # missing_base, if possible in one query
   end
 
   def export
@@ -27,4 +35,11 @@ class Default::UnitsController < ApplicationController
 
   def destroy
   end
+
+  private
+
+  def find_unit(user)
+    @unit = Unit.find_by!(id: params[:id], user: user)
+    @base = Unit.find_by!(symbol: @unit.base.symbol, user: user? ? nil : user) if @unit.base
+  end
 end