Syntax fixes for DB migration

This is a combination of [a comment from @sankage] and [a gist by
@colinpetruno]. Many thanks to them for sharing their discoveries.

[a comment from @sankage]: https://github.com/thoughtbot/paperclip/pull/2568#issuecomment-381629629
[a gist by @colinpetruno]: https://gist.github.com/colinpetruno/037de4fafa4cff695b1d7905cd6fd7c2

MIGRATING.md

@@ -132,18 +132,19 @@ class ConvertToActiveStorage < ActiveRecord::Migration[5.2]
     # sqlite
     # sqlite
     # get_blob_id = 'LAST_INSERT_ROWID()'
     # get_blob_id = 'LAST_INSERT_ROWID()'
 
 
-    active_storage_blob_statement = ActiveRecord::Base.connection.raw_connection.prepare(<<-SQL)
+    active_storage_blob_statement = ActiveRecord::Base.connection.raw_connection.prepare('active_storage_blob_statement', <<-SQL)
       INSERT INTO active_storage_blobs (
       INSERT INTO active_storage_blobs (
         `key`, filename, content_type, metadata, byte_size, checksum, created_at
         `key`, filename, content_type, metadata, byte_size, checksum, created_at
-      ) VALUES (?, ?, ?, '{}', ?, ?, ?)
+      ) VALUES ($1, $2, $3, '{}', $4, $5, $6)
     SQL
     SQL
 
 
-    active_storage_attachment_statement = ActiveRecord::Base.connection.raw_connection.prepare(<<-SQL)
+    active_storage_attachment_statement = ActiveRecord::Base.connection.raw_connection.prepare('active_storage_attachment_statement', <<-SQL)
       INSERT INTO active_storage_attachments (
       INSERT INTO active_storage_attachments (
         name, record_type, record_id, blob_id, created_at
         name, record_type, record_id, blob_id, created_at
-      ) VALUES (?, ?, ?, #{get_blob_id}, ?)
+      ) VALUES ($1, $2, $3, #{get_blob_id}, $4)
     SQL
     SQL
 
 
+    Rails.application.eager_load!
     models = ActiveRecord::Base.descendants.reject(&:abstract_class?)
     models = ActiveRecord::Base.descendants.reject(&:abstract_class?)
 
 
     transaction do
     transaction do
@@ -154,26 +155,37 @@ class ConvertToActiveStorage < ActiveRecord::Migration[5.2]
           end
           end
         end.compact
         end.compact
 
 
+        if attachments.blank?
+          next
+        end
+
         model.find_each.each do |instance|
         model.find_each.each do |instance|
           attachments.each do |attachment|
           attachments.each do |attachment|
-            active_storage_blob_statement.execute(
+            if instance.send(attachment).path.blank?
+              next
+            end
+
+            ActiveRecord::Base.connection.execute_prepared(
+              'active_storage_blob_statement', [
                 key(instance, attachment),
                 key(instance, attachment),
                 instance.send("#{attachment}_file_name"),
                 instance.send("#{attachment}_file_name"),
                 instance.send("#{attachment}_content_type"),
                 instance.send("#{attachment}_content_type"),
                 instance.send("#{attachment}_file_size"),
                 instance.send("#{attachment}_file_size"),
                 checksum(instance.send(attachment)),
                 checksum(instance.send(attachment)),
                 instance.updated_at.iso8601
                 instance.updated_at.iso8601
-            )
-
-            active_storage_attachment_statement.
-              execute(attachment, model.name, instance.id, instance.updated_at.iso8601)
+              ])
+
+            ActiveRecord::Base.connection.execute_prepared(
+              'active_storage_attachment_statement', [
+                attachment,
+                model.name,
+                instance.id,
+                instance.updated_at.iso8601,
+              ])
           end
           end
         end
         end
       end
       end
     end
     end
-
-    active_storage_attachment_statement.close
-    active_storage_blob_statement.close
   end
   end
 
 
   def down
   def down