reserve index name chars for internal rails operations

Some adapter (SQLite3) need to perform renaming operations to support
the rails DDL. These rename prefixes operate with prefixes. When an
index name already uses up the full space provieded by
`index_name_length` these internal operations will fail. This patch
introduces `allowed_index_name_length` which respects the amount of
characters used for internal operations. It will always be <=
`index_name_length` and every adapter can define how many characters
need to be reserved.

3 files changed, 22 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/database_limits.rb b/activerecord/lib/active_record/connection_adapters/abstract/database_limits.rb
index 30ccb8f0a4..2859fb31e8 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/database_limits.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/database_limits.rb
@@ -17,6 +17,15 @@ module ActiveRecord
         64
       end
 
+      # Returns the maximum allowed length for an index name. This
+      # limit is enforced by rails and Is less than or equal to
+      # <tt>index_name_length</tt>. The gap between
+      # <tt>index_name_length</tt> is to allow internal rails
+      # opreations to use prefixes in temporary opreations.
+      def allowed_index_name_length
+        index_name_length
+      end
+
       # Returns the maximum length of an index name.
       def index_name_length
         64
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index cdc8433185..2c0a18fd01 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -647,10 +647,11 @@ module ActiveRecord
           index_name   = index_name(table_name, column: column_names)
 
           if Hash === options # legacy support, since this param was a string
-            options.assert_valid_keys(:unique, :order, :name, :where, :length)
+            options.assert_valid_keys(:unique, :order, :name, :where, :length, :internal)
 
             index_type = options[:unique] ? "UNIQUE" : ""
             index_name = options[:name].to_s if options.key?(:name)
+            max_index_length = options.fetch(:internal, false) ? index_name_length : allowed_index_name_length
 
             if supports_partial_index?
               index_options = options[:where] ? " WHERE #{options[:where]}" : ""
@@ -665,10 +666,11 @@ module ActiveRecord
             end
 
             index_type = options
+            max_index_length = allowed_index_name_length
           end
 
-          if index_name.length > index_name_length
-            raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' is too long; the limit is #{index_name_length} characters"
+          if index_name.length > max_index_length
+            raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' is too long; the limit is #{max_index_length} characters"
           end
           if index_name_exists?(table_name, index_name, false)
             raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' already exists"
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index 91444950be..1b93af5033 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -187,6 +187,13 @@ module ActiveRecord
         true
       end
 
+      # Returns 51. SQLite supports index names up to 64
+      # characters. The rest is used by rails internally to perform
+      # temporary rename operations
+      def allowed_index_name_length
+        index_name_length - 13
+      end
+
       def native_database_types #:nodoc:
         {
           :primary_key => default_primary_key_type,
@@ -559,7 +566,7 @@ module ActiveRecord
 
             unless columns.empty?
               # index name can't be the same
-              opts = { name: name.gsub(/(^|_)(#{from})_/, "\\1#{to}_") }
+              opts = { name: name.gsub(/(^|_)(#{from})_/, "\\1#{to}_"), internal: true }
               opts[:unique] = true if index.unique
               add_index(to, columns, opts)
             end