Extract image field into a separate gem

[c-lliope]

Feature:

When I have a custom field type that I want to share with others,
I want to reference an official template for a field gem
so I can more easily package my field for sharing.

Implementation:

Create a template for field plugins
by extracting the image field into its own gem.

This is a breaking change with an easy fix -
anybody who uses Image fields
will need to add gem "administrate-field-image" to their Gemfile.

The source for the image field gem is up at
https://github.com/graysonwright/administrate-field-image.

[c-lliope]

@danbee, @mcmire - thoughts?

[c-lliope]

@jferris mind if I get your eyes? I'm having some trouble figuring out the right strategy to publish this feature.

This repository contains both the administrate gem and the demo application. The demo application uses images, which means that its Gemfile will be dependent on the new administrate-field-image gem.

In turn, the administrate-field-image gem is dependent on the administrate gem in this repository. This creates a little bit of a chicken-and-egg problem for publishing both of those gems.

Here's the best plan I can come up with:

• Release this branch as a new version of the administrate gem, as version 0.2.0.rc1
• Make administrate-field-image dependent on "administrate", "0.2.0.rc1", "~> 0.2.0"
• Release administrate-field-image v 0.0.1
• Make the demo app's Gemfile dependent on "administrate-field-image", "~> 0.0"
• Re-push this branch to Github to run CircleCI suite. All of the gems would be available on rubygems.org, so the suite should pass.
• Update and release this branch as version 0.2.0
• Merge branch

What do you think? Any changes that you'd make?

I'm also not sure what would happen if we set administrate-field-image dependent on "administrate", "~> 0.2.0.rc1". Do you know how the ~> operator treats release candidate versions?

Thanks for your help!

[c-lliope]

From http://guides.rubygems.org/patterns/#pessimistic-version-constraint:

Using ~> with prerelease versions will restrict you to prerelease versions only.