It is possible to have Rails automatically generate your model schema migration file using the provided model generators.

An example:

% rails generate model Post title:string
      invoke  active_record
      create    db/migrate/20220608155046_create_posts.rb
      create    app/models/post.rb
      invoke    test_unit
      create      test/models/post_test.rb
      create      test/fixtures/posts.yml

Looking at the migration file we'll see:

class CreatePosts < ActiveRecord::Migration[7.0]
  def change
    create_table :posts do |t|
      t.string :title

      t.timestamps
    end
  end
end

We can see our string title there. What happens if we try a type that makes no sense?

% rails generate model Blog title:varchar 
Could not generate field 'title' with unknown type 'varchar'.

It'll raise an error and will not generate any scaffolding.

So what are the valid types and options then?

First of all, these are the valid types:

attachment
attachments
belongs_to
boolean
date
datetime
decimal
digest
float
integer
references
rich_text
string
text
time
timestamp
token

Secondly it is possible to specify an index (which can be unique too):

% rails generate model Order number:string:index
...
class CreateOrders < ActiveRecord::Migration[7.0]
  def change
    create_table :orders do |t|
      t.string :number

      t.timestamps
    end
    add_index :orders, :number
  end
end

% rails generate model Organisation identifier:string:uniq
...
class CreateOrganisations < ActiveRecord::Migration[7.0]
  def change
    create_table :organisations do |t|
      t.string :identifier

      t.timestamps
    end
    add_index :organisations, :identifier, unique: true
  end
end

For string, text, binary and integer it is possible to set a limit:

% rails generate model Vehicle code:string{10}
...
class CreateVehicles < ActiveRecord::Migration[7.0]
  def change
    create_table :vehicles do |t|
      t.string :code, limit: 10

      t.timestamps
    end
  end
end

For decimal it is possible to set the precision and scale:

% rails generate model Product price:decimal{19.4}
...
class CreateProducts < ActiveRecord::Migration[7.0]
  def change
    create_table :products do |t|
      t.decimal :price, precision: 19, scale: 4

      t.timestamps
    end
  end
end

For references and belongs_to it is possible to indicate that it is a polymorphic association:

% rails generate model Comment commentable:references{polymorphic}
...
class CreateComments < ActiveRecord::Migration[7.0]
  def change
    create_table :comments do |t|
      t.references :commentable, polymorphic: true, null: false

      t.timestamps
    end
  end
end