#NAME

SQL::Translator::Diff - determine differences between two schemas

#DESCRIPTION

Takes two input SQL::Translator::Schemas (or SQL files) and produces ALTER statements to make them the same

#SNYOPSIS

Simplest usage:

use SQL::Translator::Diff;
my $sql = SQL::Translator::Diff::schema_diff($source_schema, 'MySQL', $target_schema, 'MySQL', $options_hash)

OO usage:

use SQL::Translator::Diff;
my $diff = SQL::Translator::Diff->new({
  output_db     => 'MySQL',
  source_schema => $source_schema,
  target_schema => $target_schema,
  %$options_hash,
})->compute_differences->produce_diff_sql;

#OPTIONS

#ignore_index_names: Match indexes based on types and fields, ignoring name.
#ignore_constraint_names: Match constrains based on types, fields and tables, ignoring name.
#output_db: Which producer to use to produce the output.
#case_insensitive: Ignore case of table, field, index and constraint names when comparing
#no_batch_alters: Produce each alter as a distinct ALTER TABLE statement even if the producer supports the ability to do all alters for a table as one statement.
#ignore_missing_methods: If the diff would need a method that is missing from the producer, just emit a comment showing the method is missing, rather than dieing with an error
#sqlt_args: Hash of extra arguments passed to "new" in SQL::Translator and the below "PRODUCER FUNCTIONS".

#PRODUCER FUNCTIONS

The following producer functions should be implemented for completeness. If any of them are needed for a given diff, but not found, an error will be thrown.

alter_create_constraint($con, $args)
alter_drop_constraint($con, $args)
alter_create_index($idx, $args)
alter_drop_index($idx, $args)
add_field($fld, $args)
alter_field($old_fld, $new_fld, $args)
rename_field($old_fld, $new_fld, $args)
drop_field($fld, $args)
alter_table($table, $args)
drop_table($table, $args)
rename_table($old_table, $new_table, $args) (optional)
batch_alter_table($table, $hash, $args) (optional)

If the producer supports batch_alter_table, it will be called with the table to alter and a hash, the keys of which will be the method names listed above; values will be arrays of fields or constraints to operate on. In the case of the field functions that take two arguments this will appear as an array reference.

I.e. the hash might look something like the following:
```
{
  alter_create_constraint => [ $constraint1, $constraint2 ],
  add_field   => [ $field ],
  alter_field => [ [$old_field, $new_field] ]
}
```
preprocess_schema($schema) (optional)

preprocess_schema is called by the Diff code to allow the producer to normalize any data it needs to first. For example, the MySQL producer uses this method to ensure that FK constraint names are unique.

Basicaly any changes that need to be made to produce the SQL file for the schema should be done here, so that a diff between a parsed SQL file and (say) a parsed DBIx::Class::Schema object will be sane.

(As an aside, DBIx::Class, for instance, uses the presence of a preprocess_schema function on the producer to know that it can diff between the previous SQL file and its own internal representation. Without this method on th producer it will diff the two SQL files which is slower, but known to work better on old-style producers.)

#AUTHOR

Original Author(s) unknown.

Refactor/re-write and more comprehensive tests by Ash Berlin ash@cpan.org.