Avoid purging the test database when loading a schema:

- Fix rails#50672
- Closes rails#50946
- ### Problem

  In adb64db, we added a change to purge the test database whenever
  the `load:schema:<name>` was run.
  This change meant that in order for the schema to load, 1) the test
  database configuration needs to be setup and 2) can be connected
  to.
  This creates some issues for users if either of both conditions
  aren't met. (For 1. it can be argued that it's a user error, but
  for 2., it's likely that a production environment can't connect
  to the test database).

  ### Details

  I dug to understand why loading a specific schema requires purging
  the test database (whereas loading all schema doesn't), but couldn't
  really see anything obvious, so I concluded that it may just be some
  overly defensive code.

  ### Solution

  Whenever we run a test, Rails maintains all test databases schema,
  and if a schema changes, the equivalent of a purge will be executed
  on all test databases.
  https://github.com/rails/rails/blob/b132744e22de64eb530d6ac407fa4b6b4a926143/railties/lib/rails/testing/maintain_test_schema.rb#L5

  I couldn't really think of a negative behaviour difference between
  purging the test database eagerly when we load the schema and lazily
  when we run the tests, so I simply removed the `purge` rake task
  dependency on `schema:load:<name>`.

Author: Edouard-chin

activerecord/lib/active_record/railties/databases.rake

@@ -474,7 +474,7 @@ db_namespace = namespace :db do
     namespace :load do
       ActiveRecord::Tasks::DatabaseTasks.for_each(databases) do |name|
         desc "Load a database schema file (either db/schema.rb or db/structure.sql, depending on configuration) into the #{name} database"
-        task name => "db:test:purge:#{name}" do
+        task name => [:load_config, :check_protected_environments] do
           ActiveRecord::Tasks::DatabaseTasks.with_temporary_pool_for_each(name: name) do |pool|
             db_config = pool.db_config
             ActiveRecord::Tasks::DatabaseTasks.load_schema(db_config, ENV["SCHEMA_FORMAT"] || db_config.schema_format)

railties/test/application/rake/multi_dbs_test.rb

@@ -510,6 +510,45 @@ def generate_models_for_animals
         end
       end
 
+      test "db:schema:load:name doesn't purge the test database. The test schema is maintained when running tests" do
+        require "#{app_path}/config/environment"
+        Dir.chdir(app_path) do
+          generate_models_for_animals
+
+          File.open("test/models/dog_test.rb", "w") do |file|
+            file.write(<<~EOS)
+              require "test_helper"
+
+              class DogTest < ActiveSupport::TestCase
+                test "Dog name type" do
+                  puts Dog.type_for_attribute(:name).type
+                end
+              end
+            EOS
+          end
+
+          rails("db:migrate:primary", "db:migrate:animals")
+
+          development_runner_output = rails("runner", "puts Dog.type_for_attribute(:name).type")
+          assert_match(/string/, development_runner_output)
+
+          test_output = rails("test", "test/models/dog_test.rb")
+          assert_match(/string/, test_output)
+
+          # Simulate a schema change
+          content = File.read("db/animals_schema.rb")
+          content.gsub!(/t\.string "name"/, "t.text \"name\"")
+          File.write("db/animals_schema.rb", content)
+
+          rails("db:schema:load:animals")
+          development_runner_output = rails("runner", "puts Dog.type_for_attribute(:name).type")
+          assert_match(/text/, development_runner_output)
+
+          test_output = rails("test", "test/models/dog_test.rb")
+          assert_match(/text/, test_output)
+        end
+      end
+
       test "db:migrate respects timestamp ordering across databases" do
         require "#{app_path}/config/environment"
         app_file "db/migrate/01_one_migration.rb", <<-MIGRATION