为什么Composer安装不同的包？

Question

I've been working with the Magento Firegento custom Composer installer, and I ran into this odd bit of Composer behavior I don't understand.

Consider the following dead simple composer.json file

{
    "require": {
        "magento-hackathon/magento-composer-installer": "*"
    }
}

If I run compser.phar install with this composer.json file, I get the following.

$ composer.phar install --no-dev 
Loading composer repositories with package information
Installing dependencies
  - Installing aoepeople/composer-installers (v0.0.1)
    Loading from cache

Writing lock file
Generating autoload files

From my mostly lay-person's understanding composer.phar, I've said

Hey, composer, please install the magento-hackathon/magento-composer-installer package from packagist.org

And composer has said back to me

Sir, yes sir! Here's the aoepeople/composer-installers package

I don't understand why composer installed aoepeople/composer-installers, when I asked for magento-hackathon/magento-composer-installer.

To be clear: I understand the reason magento-hackathon/magento-composer-installer wasn't installed is this is a package that lives in a different composer repository. My original mistake was not including this repository in my composer.json file.

However, it doesn't make sense to me that composer would install a different package than the one I asked for. When I search packagist there's no magento-hackathon/magento-composer-installer extension.

Why does packagist install a different extension? What's happening behind the scenes to make magento-hackathon/magento-composer-installer resolve to aoepeople/composer-installers? How/where in the composer source could I debug this sort of thing myself in the future?

Answer 1

Vinai published a great writeup on Magento and Composer here.

Quoting from there

Composer will use packagist.org to look where to get the libraries. Because Magento modules aren’t listed there, you will have to add the packages.firegento.org repository to the configuration as follows [...]

So you will need the repository

   "repositories":[
     {
       "type":"composer",
       "url":"http://packages.firegento.com"
     }
   ],

to get the magento composer installer.

And yes composer offers replacements. On the packaging entry for aoepeople/composer-installers you will notice the replaces section:

And since magento-hackathon/magento-composer-installer is not available on packagist composer will deliver you aoepeople/composer-installers.