Fix `PG::ConnectionBad` error when running fixtures

At first this appeared to be a multi-db bug but after some invesitgation
it was clear that this can occur just by calling `establish_connection`
from ApplicationRecord.

After some investigation we found that this only occurred when using
fixtures. The console boots fine, the server runs fine, and the tests
even run fine if we used paralellization or eager loading in the tests.

I tracked the issue down to the line that calls
`self.connection_specification_name = name` in the SchemaMigration
changes for Rails 6.0. But how can this be? That is not that major of a
change? How could `connection_specification_name` be a problem?

First `connection_specification_name` caches the name of the connection
specificatio. Second, fixtures were incorrectly holding onto a reference
to that connection.

So when you went to run the tests the models wouldn't be connected and
when the fixtures tried to load the data it would choke on that
unconnected database.

The changes here move the connection into a lambda so we can call it
when we need it rather than blowing up before the model is connected.

Fixes #36743

Co-authored-by: Aaron Patterson <aaron.patterson@gmail.com>

1 files changed, 11 insertions, 6 deletions

diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index 3df4b3c87f..e4b958f1b9 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/activerecord/lib/active_record/fixtures.rb
@@ -553,15 +553,15 @@ module ActiveRecord
         end
       end
 
-      def create_fixtures(fixtures_directory, fixture_set_names, class_names = {}, config = ActiveRecord::Base)
+      def create_fixtures(fixtures_directory, fixture_set_names, class_names = {}, config = ActiveRecord::Base, &block)
         fixture_set_names = Array(fixture_set_names).map(&:to_s)
         class_names = ClassCache.new class_names, config
 
         # FIXME: Apparently JK uses this.
-        connection = block_given? ? yield : ActiveRecord::Base.connection
+        connection = block_given? ? block : lambda { ActiveRecord::Base.connection }
 
         fixture_files_to_read = fixture_set_names.reject do |fs_name|
-          fixture_is_cached?(connection, fs_name)
+          fixture_is_cached?(connection.call, fs_name)
         end
 
         if fixture_files_to_read.any?
@@ -571,9 +571,9 @@ module ActiveRecord
             class_names,
             connection,
           )
-          cache_fixtures(connection, fixtures_map)
+          cache_fixtures(connection.call, fixtures_map)
         end
-        cached_fixtures(connection, fixture_set_names)
+        cached_fixtures(connection.call, fixture_set_names)
       end
 
       # Returns a consistent, platform-independent identifier for +label+.
@@ -612,7 +612,11 @@ module ActiveRecord
 
         def insert(fixture_sets, connection) # :nodoc:
           fixture_sets_by_connection = fixture_sets.group_by do |fixture_set|
-            fixture_set.model_class&.connection || connection
+            if fixture_set.model_class
+              fixture_set.model_class.connection
+            else
+              connection.call
+            end
           end
 
           fixture_sets_by_connection.each do |conn, set|
@@ -623,6 +627,7 @@ module ActiveRecord
                 table_rows_for_connection[table].unshift(*rows)
               end
             end
+
             conn.insert_fixtures_set(table_rows_for_connection, table_rows_for_connection.keys)
 
             # Cap primary key sequences to max(pk).