[close #23009] Limit key length

Mysql has a weird bug where it cannot index a string column of utf8mb4 if it is over a certain character limit. To get compatibility with msql we can add a limit to the key column. 191 characters is a very long key, it seems reasonable to limit across all adapters since using a longer key wouldn't be supported in mysql.

Thanks to @kamipo for the original PR and the test refactoring.

Conversation: https://github.com/rails/rails/pull/23009#issuecomment-171416629

1 files changed, 7 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/internal_metadata.rb b/activerecord/lib/active_record/internal_metadata.rb
index e5c6e5c885..10fee4dca2 100644
--- a/activerecord/lib/active_record/internal_metadata.rb
+++ b/activerecord/lib/active_record/internal_metadata.rb
@@ -5,6 +5,10 @@ module ActiveRecord
   # This class is used to create a table that keeps track of values and keys such
   # as which environment migrations were run in.
   class InternalMetadata < ActiveRecord::Base # :nodoc:
+    # Keys in mysql are limited to 191 characters, due to this no adapter can
+    # use a longer key
+    KEY_LIMIT = 191
+
     class << self
       def primary_key
         "key"
@@ -34,10 +38,11 @@ module ActiveRecord
       def create_table
         unless table_exists?
           connection.create_table(table_name, id: false) do |t|
-            t.column :key,   :string
+            t.column :key,   :string, null: false, limit: KEY_LIMIT
             t.column :value, :string
-            t.timestamps
             t.index  :key, unique: true, name: index_name
+
+            t.timestamps
           end
         end
       end