Postgres examples

[peterbecich]

This PR adds two executables to the Postgres Pagila example.

1. cabal run prints the migration to the screen to help understanding.

$ cabal run
Migration steps:
Initial commit
Add film actor, inventory, rental table
-------------
For each migration step, the sequence of SQL scripts:
Initial commit
SQL command type: PgCommandTypeDdl
SQL script: 
CREATE TABLE "actor" ("actor_id" SERIAL, "first_name" VARCHAR(45) NOT NULL, "last_name" VARCHAR(45) NOT NULL UNIQUE, "last_update" TIMESTAMP DEFAULT NOW() NOT NULL, PRIMARY KEY("actor_id")) 
SQL command type: PgCommandTypeDdl
SQL script: 
CREATE TABLE "address" ("address_id" SMALLSERIAL, "address" VARCHAR(50) NOT NULL, "address2" VARCHAR(50), "district" VARCHAR(20) NOT NULL, "city_id" SMALLINT NOT NULL, "postal_code" VARCHAR(10), "phone" VARCHAR(20) NOT NULL, "last_update" TIMESTAMP DEFAULT NOW() NOT NULL, PRIMARY KEY("address_id")) 
SQL command type: PgCommandTypeDdl
SQL script: 
CREATE TABLE "city" ("city_id" SMALLINT, "city" VARCHAR(50) NOT NULL, "country_id" SMALLINT NOT NULL, "last_update" TIMESTAMP DEFAULT NOW() NOT NULL, PRIMARY KEY("city_id")) 
SQL command type: PgCommandTypeDdl
SQL script: 
CREATE TABLE "country" ("country_id" SMALLINT, "country" VARCHAR(50) NOT NULL, "last_update" TIMESTAMP DEFAULT NOW() NOT NULL, PRIMARY KEY("country_id")) 
.
.
.
.

2. cabal run pagila-migration
   This applies the migration to a Postgres instance. It will overwrite the data in this Postgres instance. This is documented: https://github.com/haskell-beam/beam/blob/867fc8ae1ebbcc3b5832300a7492516902af3206/beam-postgres/examples/readme.md#pagila-example

The migration example will generate countries and staff and insert them into the database after migration V0001. Then V0002 migration is applied and the staff are migrated to V0002.NewStaff.

[peterbecich]

@LaurentRDC , this PR makes the example schema usable. The user must provide the instance of Postgres to target.

It also generates random data to populate the database with rows. There is a safety check for the user before any change is applied to the database.

[peterbecich]

@LaurentRDC please review, can I help in any way? Do you want the data generation removed?

[LaurentRDC]

As mentioned in the documentation for backendMigrationStepsScript, why not use displaySyntax, which has type Syntax -> String?

Recall that BeamSqlBackendSyntax Postgres is PgSyntax, and PgSyntax has an instance of Sql92DisplaySyntax such that displaySyntax will work

[peterbecich]

I need more time to understand this, can it be moved to a follow-up issue?

[LaurentRDC]

Iinsist on using displaySyntax. We want to show the appropriate, canonical way to do things in this example.

[LaurentRDC]

In the future, if there are more executables, cabal run won't be able to determine which executable is implied.

From the root of the repository, you refer users to run cabal run pagila instead of cabal run

[peterbecich]

You're right, it should be cabal run pagila-print. Fixed. There is no target for cabal run.

[LaurentRDC]

Hi Peter, I started the review weeks ago, but apparently never pressed the "finish review" button. Sorry!

[peterbecich]

Thank you @LaurentRDC

[LaurentRDC]

Can you mention backendMigrationScript in the docstring for backendMigrationStepsScript, and vice-versa? This way, users can more easily navigate the documentation

[LaurentRDC]

I don't think deriving strategies are used anymore in this changeset

[LaurentRDC]

I think you mean cabal run pagila-print to see a rendering of ....