Implement CASE Conditional Expression

[felixbuenemann]

This implements the simple and extended variant of the SQL CASE conditional:

table = Table.new(:users)

# Simple variant with a single test case:
simple_case = Arel::Nodes::Case.new table[:name]
simple_case.when('foo').then(1).when('bar').then(2).else(0).to_sql
# => CASE "users"."name" WHEN 'foo' THEN 1 WHEN 'bar' THEN 2 ELSE 0 END

# Extended variant with multiple test cases:
extended_case = Arel::Nodes::Case.new
  .when(table[:name].eq('foo')).then(1)
  .when(table[:name].eq('bar')).then(2)
  .else(0).to_sql
# => CASE WHEN "users"."name" = 'foo' THEN 1 WHEN "users"."name" = 'bar' THEN 2 ELSE 0 END

# Simple variant using predicate chaining:
table[:name].when('foo').then(1).when('bar').then(2).else(0).to_sql
# => CASE "users"."name" WHEN 'foo' THEN 1 WHEN 'bar' THEN 2 ELSE 0 END

Both variants are supported by at least PostgreSQL, MySQL, SQLite and Oracle.

This resolves #168 and is based on original code from acts_as_ordered_tree. I've asked @take-five for permission and license of the code in take-five/acts_as_ordered_tree#34.

I'm not familiar with the arel code base, so please check if there are enough tests and if my usage of build_quoted is OK (using the node's api without it felt weird).

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @tenderlove (or someone else) soon.

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[felixbuenemann]

@rafaelfranca Do you mind taking a look? Seems @tenderlove is busy.

[rafaelfranca]

Code looks good but could you change the test syntax to use assert?

[felixbuenemann]

@rafaelfranca Updated to use assert/refute syntax and rebased on master.

[kbrock]

@felixbuenemann may want to just kick this. it looks like it should work

[felixbuenemann]

Seems Travis CI fixed the bundler problem, so I removed the fix and rebased on current master. (Deleted comments regarding unrelated Travis CI failures on jruby-head.)

[pschambacher]

Love it! Thanks :smile: