Hi Sarah,
I think you can find a lot of information here :
http://rdoc.info/github/rails/arel/master/Arel/Table
What you are using here is ActiveRecord, ActiveRecord is using Arel
itself to generate it's queries.
You can use Arel directly this way :
scope :with_or, lambda {
b = Person.arel_table
where(
b[:id].in([1,2,8])
.or(
b[:id].in([5,7,9]))
)
}
This scope is mostly useless, but the main idea is here.
Sarah Allen wrote in post #1012801:
> I have a real-world application with some complex queries that I want to
> convert to Arel (as part of an upgrade to Rails 3.1). So that I can
> understand what I'm doing before I flail around in my real app, I wrote
> a little sample app (just the models) with some similar associations --
> one table joined with itself and more tables that join to another table,
> so there are some queries that require disambiguation of column names.
> I'm having trouble finding good docs that cover these kind of use case.
> Please feel free to point me to some if they exist.
>
> My sample app has a person, which has many tweets. Every person speaks
> a language and may tweet in their own language or a different language,
> so there's a language table associated with both people and tweets.
> Also, people have followers and may be followed by other people (stored
> in the same table, of course). I posted the app on github with some
> seed data if anyone wants to try it in the console:
>
> git clone git://github.com/ultrasaurus/twitter_like_example_app.git
> sample_app
> cd sample_app
> bundle install
> rake db:migrate
> rake db:seed
> rails c
>
> I can create simple queries like this:
>
> list all people's names alphabetically
>> Person.order(:name).all.map(&:name)
>
> how many people speak french?
>> Person.joins(:language).where(:languages => {:code => 'fr'}).count
>
> How many German people are in the data set?
>> Person.where(:language_id => 2).order(:name).count
>
> Create a list of them alphabetized by name
>> Person.where(:language_id => 2).order(:name).all.map(&:name)
>
> How many people have the first name that begins with "A"?
>> Person.where("name like 'A%'").count
>
> I'm not having as much success with more complicated ones like these:
>
> all people grouped by language, then alphabetized by name
> this sorts people by name not language:
>> Person.order(:name, {:language => :english_name}).all.map { |p| puts
> "#{p.name} #{p.language.english_name}" }
>
> how many people tweet in french?
>> Person.where(:tweets => {:language_id => 'fr'}).count
> (0.1ms) SELECT COUNT(*) FROM "people" WHERE "tweets"."language_id" =
> 0
> SQLite3::SQLException: no such column: tweets.language_id: SELECT
> COUNT(*) FROM "people" WHERE "tweets"."language_id" = 0
> ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column:
> tweets.language_id: SELECT COUNT(*) FROM "people" WHERE
> "tweets"."language_id" = 0
>
> How many people tweet in french who are english speakers?
>
> all tweets sorted by language (with language alphabetized)
>
> How many people with a first name beginning with A follow someone whose
> first name begins with "S" ?
>
> List all the tweets in in french that can be seen by french people (e.g.
> where the person whose language is french follows someone who has a
> tweet which is french)
>
>
> Thanks in advance,
> Sarah
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