Allow idempotent database registration for Rails reloading (#147)

* Allow idempotent database registration for Rails reloading

When using Rails' code reloading in development, the `to_prepare`
callback is executed multiple times, causing Nandi to attempt
re-registering databases. This would previously fail with
"Database already registered" error.

This change allows re-registration of databases when the configuration
is identical to the existing one, making the registration idempotent.
This is particularly important for Rails applications using the
`Rails.application.reloader.to_prepare` pattern.

The change maintains backward compatibility by still raising an error
when attempting to register a database with a different configuration.

Co-Authored-By: Claude <[email protected]>

* Bump version to 2.0.1 for bug fix release

---------

Co-authored-by: Claude <[email protected]>

Author: mfmmq

lib/nandi/multi_database.rb

@@ -54,13 +54,14 @@ class Database
       # @return [String]
       attr_accessor :output_directory
 
-      attr_reader :name, :default
+      attr_reader :name, :default, :raw_config
 
       attr_accessor :migration_directory,
                     :lockfile_name
 
       def initialize(name:, config:)
         @name = name
+        @raw_config = config
         @default = @name == :primary || config[:default] == true
 
         # Paths and files
@@ -115,6 +116,10 @@ def default
 
     def register(name, config)
       name = name.to_sym
+
+      # Allow re-registration with identical config (for Rails reloading)
+      return @databases[name] if @databases.key?(name) && @databases[name].raw_config == config
+
       raise ArgumentError, "Database #{name} already registered" if @databases.key?(name)
 
       @databases[name] = Database.new(name: name, config: config)

lib/nandi/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Nandi
-  VERSION = "2.0.0"
+  VERSION = "2.0.1"
 end

spec/nandi/multi_database_spec.rb

@@ -54,12 +54,25 @@
       expect(multi_db.default.name).to eq(:primary)
     end
 
-    it "raises error for duplicate database registration" do
+    it "raises error for duplicate database registration with different config" do
       expect do
         multi_db.register(:primary, migration_directory: "db/new")
       end.to raise_error(ArgumentError, "Database primary already registered")
     end
 
+    it "allows re-registration with identical config (for Rails reloading)" do
+      original_config = { migration_directory: "db/safe_migrations", output_directory: "db/migrate" }
+
+      # First registration
+      db1 = multi_db.register(:reloadable, original_config)
+
+      # Re-registration with same config should return the same database object
+      db2 = multi_db.register(:reloadable, original_config)
+
+      expect(db2).to eq(db1)
+      expect(multi_db.names.count(:reloadable)).to eq(1)
+    end
+
     it "converts string names to symbols" do
       multi_db.register("string_name", migration_directory: "db/string")
       expect(multi_db.names).to include(:string_name)