Ensure `connects_to` can only be called on base or abstract classes

`connectes_to` should only be called on `ActiveRecord::Base` or abstract
classes. This is recommended in the documentation but until now was not
enforced by the code. It's unsafe to open too many connections to mysql
(and probably other databases), so it's safest to have 1 class for the
connection and subclass from that.
Co-authored-by: NJohn Crepezzi <john.crepezzi@gmail.com>

activerecord/CHANGELOG.md

+*   `connects_to` can only be called on `ActiveRecord::Base` or abstract classes.
+
+    Ensure that `connects_to` can only be called from `ActiveRecord::Base` or abstract classes. This protects the application from opening duplicate or too many connections.
+
+    *Eileen M. Uchitelle*, *John Crepezzi*
+
 *   All connection adapters `execute` now raises `ActiveRecord::ConnectionNotEstablished` rather than
     `ActiveRecord::InvalidStatement` when they encounter a connection error.
 

activerecord/lib/active_record/connection_handling.rb

@@ -79,6 +79,8 @@ def establish_connection(config_or_env = nil)
     #
     # Returns an array of database connections.
     def connects_to(database: {}, shards: {})
+      raise NotImplementedError, "connects_to can only be called on ActiveRecord::Base or abstract classes" unless self == Base || abstract_class?
+
       if database.present? && shards.present?
         raise ArgumentError, "connects_to can only accept a `database` or `shards` argument, but not both arguments."
       end

activerecord/test/cases/base_test.rb

@@ -1661,6 +1661,14 @@ def test_protected_environments_are_stored_as_an_array_of_string
     end
   end
 
+  test "cannot call connects_to on non-abstract or non-ActiveRecord::Base classes" do
+    error = assert_raises(NotImplementedError) do
+      Bird.connects_to(database: { writing: :arunit })
+    end
+
+    assert_equal "connects_to can only be called on ActiveRecord::Base or abstract classes", error.message
+  end
+
   test "cannot call connected_to on subclasses of ActiveRecord::Base" do
     error = assert_raises(NotImplementedError) do
       Bird.connected_to(role: :reading) { }

activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb

@@ -24,7 +24,11 @@ def teardown
         clean_up_connection_handler
       end
 
-      class MultiConnectionTestModel < ActiveRecord::Base
+      class SecondaryBase < ActiveRecord::Base
+        self.abstract_class = true
+      end
+
+      class MultiConnectionTestModel < SecondaryBase
       end
 
       def test_multiple_connection_handlers_works_in_a_threaded_environment
@@ -33,7 +37,7 @@ def test_multiple_connection_handlers_works_in_a_threaded_environment
 
         # We need to use a role for reading not named reading, otherwise we'll prevent writes
         # and won't be able to write to the second connection.
-        MultiConnectionTestModel.connects_to database: { writing: { database: tf_writing.path, adapter: "sqlite3" }, secondary: { database: tf_reading.path, adapter: "sqlite3" } }
+        SecondaryBase.connects_to database: { writing: { database: tf_writing.path, adapter: "sqlite3" }, secondary: { database: tf_reading.path, adapter: "sqlite3" } }
 
         MultiConnectionTestModel.connection.execute("CREATE TABLE `multi_connection_test_models` (connection_role VARCHAR (255))")
         MultiConnectionTestModel.connection.execute("INSERT INTO multi_connection_test_models VALUES ('writing')")
@@ -73,7 +77,7 @@ def test_multiple_connection_handlers_works_in_a_threaded_environment
       def test_loading_relations_with_multi_db_connection_handlers
         # We need to use a role for reading not named reading, otherwise we'll prevent writes
         # and won't be able to write to the second connection.
-        MultiConnectionTestModel.connects_to database: { writing: { database: ":memory:", adapter: "sqlite3" }, secondary: { database: ":memory:", adapter: "sqlite3" } }
+        SecondaryBase.connects_to database: { writing: { database: ":memory:", adapter: "sqlite3" }, secondary: { database: ":memory:", adapter: "sqlite3" } }
 
         relation = ActiveRecord::Base.connected_to(role: :secondary) do
           MultiConnectionTestModel.connection.execute("CREATE TABLE `multi_connection_test_models` (connection_role VARCHAR (255))")

activerecord/test/cases/connection_adapters/connection_handlers_sharding_db_test.rb

@@ -264,15 +264,23 @@ def test_calling_connected_to_on_a_non_existent_shard_raises
         end
       end
 
-      class ShardConnectionTestModel < ActiveRecord::Base
+      class SecondaryBase < ActiveRecord::Base
+        self.abstract_class = true
       end
 
-      class ShardConnectionTestModelB < ActiveRecord::Base
+      class ShardConnectionTestModel < SecondaryBase
+      end
+
+      class SomeOtherBase < ActiveRecord::Base
+        self.abstract_class = true
+      end
+
+      class ShardConnectionTestModelB < SomeOtherBase
       end
 
       def test_same_shards_across_clusters
-        ShardConnectionTestModel.connects_to shards: { one: { writing: { database: ":memory:", adapter: "sqlite3" } } }
-        ShardConnectionTestModelB.connects_to shards: { one: { writing: { database: ":memory:", adapter: "sqlite3" } } }
+        SecondaryBase.connects_to shards: { one: { writing: { database: ":memory:", adapter: "sqlite3" } } }
+        SomeOtherBase.connects_to shards: { one: { writing: { database: ":memory:", adapter: "sqlite3" } } }
 
         ActiveRecord::Base.connected_to(shard: :one) do
           ShardConnectionTestModel.connection.execute("CREATE TABLE `shard_connection_test_models` (shard_key VARCHAR (255))")
@@ -287,7 +295,7 @@ def test_same_shards_across_clusters
       end
 
       def test_sharding_separation
-        ShardConnectionTestModel.connects_to shards: {
+        SecondaryBase.connects_to shards: {
           default: { writing: { database: ":memory:", adapter: "sqlite3" } },
           one: { writing: { database: ":memory:", adapter: "sqlite3" } }
         }
@@ -323,7 +331,7 @@ def test_swapping_shards_in_a_multi_threaded_environment
         tf_default = Tempfile.open "shard_key_default"
         tf_shard_one = Tempfile.open "shard_key_one"
 
-        ShardConnectionTestModel.connects_to shards: {
+        SecondaryBase.connects_to shards: {
           default: { writing: { database: tf_default.path, adapter: "sqlite3" } },
           one: { writing: { database: tf_shard_one.path, adapter: "sqlite3" } }
         }
@@ -368,7 +376,7 @@ def test_swapping_shards_and_roles_in_a_multi_threaded_environment
         tf_default_reading = Tempfile.open "shard_key_default_reading"
         tf_shard_one_reading = Tempfile.open "shard_key_one_reading"
 
-        ShardConnectionTestModel.connects_to shards: {
+        SecondaryBase.connects_to shards: {
           default: { writing: { database: tf_default.path, adapter: "sqlite3" }, secondary: { database: tf_default_reading.path, adapter: "sqlite3" } },
           one: { writing: { database: tf_shard_one.path, adapter: "sqlite3" }, secondary: { database: tf_shard_one_reading.path, adapter: "sqlite3" } }
         }