Ensure prefixes are unique

lib/prefixed_ids.rb

@@ -28,6 +28,14 @@ def self.split_id(prefix_id, delimiter = PrefixedIds.delimiter)
     [prefix, id]
   end
 
+  def self.register_prefix(prefix, model:)
+    if (existing_model = PrefixedIds.models[prefix]) && existing_model != model
+      raise Error, "Prefix #{prefix} already defined for model #{model}"
+    end
+
+    PrefixedIds.models[prefix] = model
+  end
+
   # Adds `has_prefix_id` method
   module Rails
     extend ActiveSupport::Concern
@@ -46,7 +54,7 @@ def has_prefix_id(prefix, override_find: true, override_param: true, fallback: t
         self._prefix_id_fallback = fallback
 
         # Register with PrefixedIds to support PrefixedIds#find
-        PrefixedIds.models[prefix.to_s] = self
+        PrefixedIds.register_prefix(prefix.to_s, model: self)
       end
     end
   end

test/prefixed_ids_test.rb

@@ -227,4 +227,28 @@ class PrefixedIdsTest < ActiveSupport::TestCase
       assert_equal prefix_decoded, [1, 1]
     end
   end
+
+  test "register_prefix adds the expected prefix and model" do
+    model = Class.new(ApplicationRecord) do
+      def self.name
+        "TestModel"
+      end
+    end
+
+    PrefixedIds.register_prefix("test_model", model: model)
+    assert_equal model, PrefixedIds.models["test_model"]
+  end
+
+  test "has_prefix_id raises when prefix was already used" do
+    assert PrefixedIds.models.key?("user")
+    assert_raises PrefixedIds::Error do
+      Class.new(ApplicationRecord) do
+        def self.name
+          "TestModel"
+        end
+
+        has_prefix_id :user
+      end
+    end
+  end
 end