Move SchemaMigration to migration_context

This PR moves the `schema_migration` to `migration_context` so that we
can access the `schema_migration` per connection.

This does not change behavior of the SchemaMigration if you are using
one database. This also does not change behavior of any public APIs.
`Migrator` is private as is `MigrationContext` so we can change these as
needed.

We now need to pass a `schema_migration` to `Migrator` so that we can
run migrations on the right connection outside the context of a rake
task.

The bugs this fixes were discovered while debugging the issues around
the SchemaCache on initialization with multiple database. It was clear
that `get_all_versions` wouldn't work without these changes outside the
context of a rake task (because in the rake task we establish a
connection and change AR::Base.connection to the db we're running on).

Because the `SchemaCache` relies on the `SchemaMigration` information we
need to make sure we store it per-connection rather than on
ActiveRecord::Base.

[Eileen M. Uchitelle & Aaron Patterson]

1 files changed, 7 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/tasks/database_tasks.rb b/activerecord/lib/active_record/tasks/database_tasks.rb
index cf98a203bd..aecc9350e8 100644
--- a/activerecord/lib/active_record/tasks/database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/database_tasks.rb
@@ -200,9 +200,10 @@ module ActiveRecord
 
       def truncate_tables(configuration)
         ActiveRecord::Base.connected_to(database: { truncation: configuration }) do
-          table_names = ActiveRecord::Base.connection.tables
+          conn = ActiveRecord::Base.connection
+          table_names = conn.tables
           table_names -= [
-            SchemaMigration.table_name,
+            conn.schema_migration.table_name,
             InternalMetadata.table_name
           ]
 
@@ -233,7 +234,7 @@ module ActiveRecord
       end
 
       def migrate_status
-        unless ActiveRecord::SchemaMigration.table_exists?
+        unless ActiveRecord::Base.connection.schema_migration.table_exists?
           Kernel.abort "Schema migrations table does not exist yet."
         end
 
@@ -328,6 +329,7 @@ module ActiveRecord
       def dump_schema(configuration, format = ActiveRecord::Base.schema_format, spec_name = "primary") # :nodoc:
         require "active_record/schema_dumper"
         filename = dump_filename(spec_name, format)
+        connection = ActiveRecord::Base.connection
 
         case format
         when :ruby
@@ -336,9 +338,9 @@ module ActiveRecord
           end
         when :sql
           structure_dump(configuration, filename)
-          if ActiveRecord::SchemaMigration.table_exists?
+          if connection.schema_migration.table_exists?
             File.open(filename, "a") do |f|
-              f.puts ActiveRecord::Base.connection.dump_schema_information
+              f.puts connection.dump_schema_information
               f.print "\n"
             end
           end