Add schema support to Persistent (issue #93)

[curranosaurus]

Addresses #93 by adding schema support to Persistent. You specify a Persistent model's schema using syntax like schema=foo.
I worked on this together with @benjonesy.

Design challenges and decisions

"Schema" means many things to many backends

In this PR, we use the word "schema" to refer to various devices that backends offer for namespacing tables.

• ✔️ In PostgreSQL, a schema exists in between the level of a table and a database.
• ✔️ In MySQL and SQLite, schemas and databases are synonymous concepts.
• ❌ MongoDB and Redis do not have a concept analogous to schemas. This PR does not extend support to these backends.

Schemas may or may not exist

We considered designing this PR so that Persistent would create schemas for the user if they did not already exist. But we chose not to do that. This means that local development with Persistent using schemas will require some manual SQL migrations to create the schemas that the user wants. But for conventional use-cases, schema creation will be a rare event, so this is not very onerous for developers. Below are some issues one would have to solve to make Persistent automate schema creation for the developer:

Three-stage migrations

RIght now, the CREATe TABLE DDL statements are sorted to the top of the migration output, so that the (potentially recursive) foreign-key constraints are created only after all of the tables they reference exist. If we also wanted Persistent to ensure the existence of schemas, we would need to sort backend-appropriate versions of CREATE SCHEMA statements above the CREATe TABLE statements.

Granting permissions in MySQL

Before a connection can use a newly created database in MySQL, one must run a GRANT statement to give the current user the requisite DDL operations to run CREATe TABLE statements. We would somehow need to make Persistent aware of the GRANT statement required.

File name and directory in SQLite

SQLite's invocation for creating a new database also specifies the file name and location for that database's .db file. Persistent would need to be opinionated about this directory scheme, or we would have to add a configuration option, if we wanted to support schema creation in SQLite.

PR Template checklist

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock
• Ran stylish-haskell on any changed files.
• Adhered to the code style (see the .editorconfig file for details)

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[curranosaurus]

Alright, I think I made it through all of your requests, @parsonsmatt. This should be ready for another round of review. Thanks for your advice!

I think there's further work to support this on redis and mong, even if it's just mconcat [schemaName, "_", tableName]. But I'll defer that decision to someone who is using those backends.

Yeah, I suspect you're right that people will want to apply this feature to those backends. But I don't know what namespacing conventions people will expect for those data stores. So I agree that we should leave that decision to a future contributor.

[curranosaurus]

Looks like the MySQL schema setup failed:

Run mysql -utest -ptest -h127.0.0.1 --port=33306 test -e "CREATE DATABASE foo; GRANT ALL ON foo.* to 'test'@'localhost';"
  mysql -utest -ptest -h127.0.0.1 --port=33306 test -e "CREATE DATABASE foo; GRANT ALL ON foo.* to 'test'@'localhost';"
  shell: /usr/bin/bash -e {0}
  env:
    CONFIG: --enable-tests --enable-benchmarks
Warning: arning] Using a password on the command line interface can be insecure.
ERROR 1044 (42000) at line 1: Access denied for user 'test'@'%' to database 'foo'
Error: Process completed with exit code 1.

I am guessing it's because the test user doesn't have rights to create databases or grant itself access to those databases. I'll probably need to rewrite the CLI invocation to log in as root user?

[curranosaurus]

I think -uroot -ptest will work because near the top of this file there's a declaration MYSQL_ROOT_PASSWORD: test.

[curranosaurus]

mysql -uroot -ptest -h127.0.0.1 --port=33306 test -e "CREATE DATABASE foo; GRANT ALL ON foo.* to 'test'@'localhost';"
ERROR 1410 (42000) at line 1: You are not allowed to create a user with GRANT

Maybe the host isn't called localhost?

[parsonsmatt]

This may be relevant: https://stackoverflow.com/questions/50177216/how-to-grant-all-privileges-to-root-user-in-mysql-8-0

[curranosaurus]

My current theory is that the user test@<?> exists because of the MYSQL_USER: test declaration on line 32. But that instead of <?> being localhost, it should be 127.0.0.1, matching the way that we log in to the db using the mysql command.