don't try to generate foreign keys for fake primary keys, fixes #19

if a :primary_key isn't actually unique (or is insufficiently unique),
don't try to create a key, since it will fail.

lib/immigrant/key_validator.rb

@@ -6,11 +6,16 @@ def valid?(key)
       return false unless tables.include?(key.to_table)
       return false unless column_exists?(key.from_table, key.options[:column])
       return false unless column_exists?(key.to_table, key.options[:primary_key])
+      return false unless key.options[:primary_key] == "id" || has_unique_index?(key.to_table, key.options[:primary_key])
       true
     end
 
    private
 
+    def has_unique_index?(table_name, column_name)
+      connection.indexes(table_name).any? { |index| index.columns == [column_name] && index.unique && index.where.nil? }
+    end
+
     def column_exists?(table_name, column_name)
       columns_for(table_name).any? { |column| column.name == column_name }
     end

test/immigrant_test.rb

@@ -41,16 +41,21 @@ def tables
       ActiveSupport::DescendantsTracker.direct_descendants(ActiveRecord::Base).map(&:table_name)
     end
     def columns(table_name)
-      AnyColumn.new
+      AnyCollection.new
+    end
+    def indexes(table_name)
+      AnyCollection.new
     end
   end
 
-  class AnyColumn
+  class AnyCollection
     def any?
       true
     end
   end
 
+  Index = ActiveRecord::ConnectionAdapters::IndexDefinition
+
   def teardown
     subclasses = ActiveSupport::DescendantsTracker.direct_descendants(ActiveRecord::Base)
     subclasses.each do |subclass|
@@ -455,6 +460,26 @@ class Widget < ActiveRecord::Base
     end
   end
 
+  test 'insufficiently unique "primary" keys should not generate a foreign key' do
+    given <<-CODE
+      class Setting < ActiveRecord::Base; end
+
+      class SettingValue < ActiveRecord::Base
+        belongs_to :setting,
+                   :conditions => { :deleted_at => nil },
+                   foreign_key: :name, primary_key: :name
+      end
+    CODE
+
+    # emulate `"index_settings_on_name" UNIQUE, btree (name) WHERE deleted_at IS NULL`
+    # it's unique, but not enough for a foreign key just on name
+    indexes = [Index.new("settings", ["name"], true, "deleted_at IS NULL")]
+
+    MockConnection.stub_any_instance(:indexes, indexes) do
+      assert_equal([], infer_keys)
+    end
+  end
+
   test 'ignore_keys should be respected' do
     given <<-CODE
       class User <  ActiveRecord::Base; end