Relationship are not showing in the rest.

I have a post type named ‘ims_customer’ and it has relationship with another post type named ‘ims_contact’. I can perfectly get the list of ‘ims_contact’ post types and details in rest response. But I don’t get the ‘ims_contact’ post type fields when I request ‘ims_customer’ post type. It seems the relationship are not showing. How do I include the ‘ims_contact’ field in the ‘ims_customer’ response.