i want to do model validation with association in a single form. i have two tables users(parent table) and user_details(child
validation working for only users table because i have created newEntity for users table only.
That has nothing to do with it. The problem is that you are not following the naming conventions properly.
Filenames must match classnames, so it's UserDetailsTable.php, not UserDetails.php, UsersTable.php, not usersTable.php, etc.
And the correct property name for a hasOne association, is the singular, underscored variant of the association name, so for UserDetails that would be user_detail, consequently the name for the related form control should be user_detail.name.
See also