Auto-generate foreign key constraint names when not provided

[dereuromark]

Summary

• Auto-generate foreign key constraint names when not provided in addForeignKey()
• MySQL and SQLite adapters now use the pattern {table}_{columns} (matching PostgresAdapter and SqlserverAdapter behavior)

This fixes an issue where unnamed constraints could cause problems with CakePHP's ConnectionHelper::dropTables() which expects string constraint names. When no name was provided, MySQL would auto-generate one, but the schema reflection could store it with integer array keys, causing TypeError in TableSchema::getConstraint().

Fixes #1040

[dereuromark]

Updated PR #1042 with database-specific limits:

• MySQL: 61 chars (64 - 3)
• PostgreSQL: 60 chars (63 - 3)
• SQL Server: 125 chars (128 - 3)
• SQLite: No limit

[markstory]

Changing the name of indexes, will change the schema created by a migration history. What used to be automatically named by the database engine will now be given a different name by migrations which could cause index add, index drop/change operations to fail as the name of an index has changed.

I don't think we have a good solution for this, but it should be something we point out in the release notes as a potential upgrade issue.

[dereuromark]

After the merge of this and follow up, maybe I can invest some time into shimming possibilities?
Like if it doesnt find by name, maybe looking up by fields and see which one would likely match the most?
This could potentially mitigate of those issues.

  // Pseudocode for dropForeignKey fallback                                                                
  $fk = $this->findForeignKeyByName($name);                                                                
  if (!$fk) {                                                                                              
      $fk = $this->findForeignKeyByColumns($columns);                                                      
      // Log warning about name mismatch                                                                   
  }                                                                                                        

But that would only work if we can reverse engineer the columns from the up() statement.

[dereuromark]

All good?

[jamisonbryant]

Looks good, should we update the docs and the migration guide? Changing the conventional format for naming foreign keys feels like something we should warn users about.

[dereuromark]

https://github.com/cakephp/migrations/pull/1049/changes is the follow up then.