Symptoms

I'm testing application that has services that bind to core/service-user but "Users" tab is not shown in customer panel.

Cause

Relation to service user lacks the "required": "true" attribute:

 "relations": {
    "user": {
      "type": "http://aps-standard.org/types/core/service-user/1.0",
      "collection": false
    },

Resolution

Correct declaration:

 "relations": {
    "user": {
      "type": "http://aps-standard.org/types/core/service-user/1.0",
      "required": true,
      "collection": false
    },

More on properties and their attributes here.

Internal content