Description:
    Generates two new database migrations which will safely add an arbitrary
    check constraint and validate it separately.

Example:
    rails generate nandi:check_constraint foos baz_or_quux_not_null

    This will create:
        db/safe_migrations/20190424123727_add_check_constraint_baz_or_quux_not_null_on_foos.rb
        db/safe_migrations/20190424123728_validate_check_constraint_baz_or_quux_not_null_on_foos.rb

    If no --database option is specified, uses the default database.

Example:
    rails generate nandi:check_constraint foos baz_or_quux_not_null --validation-timeout 20000

    This will create:
        db/safe_migrations/20190424123727_add_check_constraint_baz_or_quux_not_null_on_foos.rb
        db/safe_migrations/20190424123728_validate_check_constraint_baz_or_quux_not_null_on_foos.rb

    The statement timeout in the second migration will be set to 20,000ms.

Multi-database examples:
    rails generate nandi:check_constraint users email_valid --database primary
        creates check constraint migrations in primary database directory

    rails generate nandi:check_constraint events status_valid --database analytics
        creates check constraint migrations in analytics database directory
