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How to introduce Darwin to existing database

You can also use Darwin in projects where a data structure already exists. By default, you create create.sql script and patch set as if the database were empty (see documentation). Then add one or more guess_version.sql scripts, which Darwin uses to determine the version of an existing data structure.

The detection procedure is as follows:

  • in case that Darwin does not have an entry in its internal table about existence of the component it'll try to find all scripts starting with guess_ word before executing any of create.sql or patch_*.sql scripts (eg. guess_1.0.sql, guess_2.1.0.sql, etc.)
  • it starts executing them in order from the lowest version to the highest version
  • last script, which does not finish with an exception (or if the guess contents starts with select count and returns a number greater than 0) will be considered determines the version of model existing in the database
  • version number is parsed from the guess_(.*?).sql file name and stored to Darwin internal table specifying component version to start with

If even first guess script fails with exception or returns number equal to zero, Darwin creates model with create.sql script.

This way, you can easily connect to an existing data structure.

Examples of the guess scripts

1. Use SQL query that must finish without exception in case model is up-to-date

Check existence of some table that was created for specific model version:

select * from SOME_TABLE

If table doesn't exist it means, that model of requested version doesn't exist in the database.

2. Use SQL count query that must return result greater that one

Check existence of some record that was inserted in specific model version:

select count(0) from SOME_TABLE where col = 'value';

If statement returns 1 it means that record is already in the table and patch with this version was already applied, if it returns 0 it means that Darwin would need to apply the patch now.

You can also query database infrastructural tables in the guess script - for example:

SELECT count(0) 
FROM information_schema.tables
WHERE table_schema = 'test' 
    AND table_name = 'testtable'