index(column_name, **options)
public

Adds a new index to the table. column_name can be a single Symbol, or an Array of Symbols.

t.index(:name)
t.index([:branch_id, :party_id], unique: true)
t.index([:branch_id, :party_id], unique: true, name: 'by_branch_party')

See {connection.add_index}[rdoc-ref:SchemaStatements#add_index] for details of the options you can use.