generate a model with a string field as primary key

No, you can't. By default the primary key is an auto-increment integer.

However, you can open the migration that was generated from the command, and change it (before running the rake db:migrate command). The migration will likely have a create_table command:

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      # ...
    end
  end
end

If you read the create_table documentation, you will notice you can pass two options. Specifically, you need to set :id to false to not generate an id field, and you will need to specify the name of the primary key field.

    create_table :users, id: false, primary_key: :email do |t|

To add to @Simone Carletti's answer, you may need to use execute to set the primary key (if it's obscure). This would be especially true if you're modifying an existing table, which you're obviously not doing:

class CreateUsers < ActiveRecord::Migration
   def change
      create_table :users, id: false do |t|
         t.string :email, null: false
         t.timestamps
      end
      execute "ALTER TABLE users ADD PRIMARY KEY (email);"
   end
end

We use uuid's in some of our apps, and that's what we had to do (primary_key: :uuid didn't work)...

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