Don't allow granular swapping for shards in legacy

This is a separate commit because I want it to be easy to revert if we
change our minds. After some discussion I think it is confusing that you
could swap shard but not role granularly in legacy mode. This change
forces users to always either have global swapping until moved off
legacy mode. This will prevent a situation where `AnimalsBase` can
change the shard granularly but the role globally.

activerecord/lib/active_record/connection_handling.rb

@@ -135,17 +135,21 @@ module ActiveRecord
     #
     # The database kwarg is deprecated and will be removed in 6.2.0 without replacement.
     def connected_to(database: nil, role: nil, shard: nil, prevent_writes: false, &blk)
-      if shard && role && (!abstract_class? || self != Base)
-        raise NotImplementedError, "calling `connected_to` with `role` and `shard` is only allowed on ActiveRecord::Base or abstract classes" unless abstract_class? || self == Base
-      elsif legacy_connection_handling && self != Base
-        raise NotImplementedError, "`connected_to` can only be called on ActiveRecord::Base"
-      elsif database
-        ActiveSupport::Deprecation.warn("The database key in `connected_to` is deprecated. It will be removed in Rails 6.2.0 without replacement.")
+      if legacy_connection_handling
+        if self != Base
+          raise NotImplementedError, "`connected_to` can only be called on ActiveRecord::Base with legacy connection handling."
+        end
+      else
+        if self != Base && !abstract_class
+          raise NotImplementedError, "calling `connected_to` is only allowed on ActiveRecord::Base or abstract classes."
+        end
       end
 
       if database && (role || shard)
         raise ArgumentError, "`connected_to` cannot accept a `database` argument with any other arguments."
       elsif database
+        ActiveSupport::Deprecation.warn("The database key in `connected_to` is deprecated. It will be removed in Rails 6.2.0 without replacement.")
+
         if database.is_a?(Hash)
           role, database = database.first
           role = role.to_sym

activerecord/test/cases/base_test.rb

@@ -1632,7 +1632,7 @@ class BasicsTest < ActiveRecord::TestCase
       Bird.connected_to(role: :reading) { }
     end
 
-    assert_equal "`connected_to` can only be called on ActiveRecord::Base", error.message
+    assert_equal "`connected_to` can only be called on ActiveRecord::Base with legacy connection handling.", error.message
   ensure
     clean_up_legacy_connection_handlers
     ActiveRecord::Base.legacy_connection_handling = old_value
@@ -1643,7 +1643,7 @@ class BasicsTest < ActiveRecord::TestCase
       Bird.connected_to(role: :reading, shard: :default) { }
     end
 
-    assert_equal "calling `connected_to` with `role` and `shard` is only allowed on ActiveRecord::Base or abstract classes", error.message
+    assert_equal "calling `connected_to` is only allowed on ActiveRecord::Base or abstract classes.", error.message
   end
 
   test "can call connected_to with role and shard on abstract classes" do

activerecord/test/cases/connection_adapters/legacy_connection_handlers_sharding_db_test.rb

@@ -308,27 +308,17 @@ module ActiveRecord
             assert_equal "primary_shard_one_replica", ActiveRecord::Base.connection_pool.db_config.name
             assert_equal "secondary_shard_one_replica", SecondaryBase.connection_pool.db_config.name
 
-            SecondaryBase.connected_to(role: :writing, shard: :default) do
-              # Note: Currently there is only granular shard swapping, not role swapping
-              # so this switches the shard only for SecondaryBase but role for both
-              assert_equal "primary_shard_one", ActiveRecord::Base.connection_pool.db_config.name
+            ActiveRecord::Base.connected_to(role: :writing, shard: :default) do
+              assert_equal "primary", ActiveRecord::Base.connection_pool.db_config.name
               assert_equal "secondary", SecondaryBase.connection_pool.db_config.name
 
-              ActiveRecord::Base.connected_to(role: :writing, shard: :default) do
-                assert_equal "primary", ActiveRecord::Base.connection_pool.db_config.name
-                assert_equal "secondary", SecondaryBase.connection_pool.db_config.name
-              end
-
-              SecondaryBase.connected_to(role: :reading, shard: :shard_two) do
-                # granular switching means only SecondaryBase uses shard_two
-                assert_equal "primary_shard_one_replica", ActiveRecord::Base.connection_pool.db_config.name
+              ActiveRecord::Base.connected_to(role: :reading, shard: :shard_two) do
+                # ActiveRecord::Base has no shard_two
                 assert_equal "secondary_shard_two_replica", SecondaryBase.connection_pool.db_config.name
               end
 
               ActiveRecord::Base.connected_to(role: :writing) do
-                # shard is inherited for AR::Base from outer most block :shard_one
-                assert_equal "primary_shard_one", ActiveRecord::Base.connection_pool.db_config.name
-                # shard is inherited for SecondaryBase by second block :default
+                assert_equal "primary", ActiveRecord::Base.connection_pool.db_config.name
                 assert_equal "secondary", SecondaryBase.connection_pool.db_config.name
               end
             end
@@ -559,13 +549,13 @@ module ActiveRecord
           shard_one_latch.count_down
         end
 
-        SecondaryBase.connected_to(shard: :one, role: :secondary) do
+        ActiveRecord::Base.connected_to(shard: :one, role: :secondary) do
           shard_default_latch.count_down
 
           assert_equal "shard_key_one_secondary", ShardConnectionTestModel.connection.select_value("SELECT shard_key from shard_connection_test_models")
-          assert_equal "shard_key_default_secondary_b", ShardConnectionTestModelB.connection.select_value("SELECT shard_key from shard_connection_test_models")
+          assert_equal "shard_key_one_secondary_b", ShardConnectionTestModelB.connection.select_value("SELECT shard_key from shard_connection_test_models")
 
-          SomeOtherBase.connected_to(shard: :one, role: :secondary) do
+          ActiveRecord::Base.connected_to(shard: :one, role: :secondary) do
             assert_equal "shard_key_one_secondary", ShardConnectionTestModel.connection.select_value("SELECT shard_key from shard_connection_test_models")
             assert_equal "shard_key_one_secondary_b", ShardConnectionTestModelB.connection.select_value("SELECT shard_key from shard_connection_test_models")
           end