Cannot populate admin_users

I have a collection named “articles”. When I populate using *, I can see all the related collection except admin_user. Any idea how to resolve this? One more Question. I want to get admin_user using api endpoint and I cannot seem to find anything about it in documentation.

Below is the schema of articles:

{

“kind”: “collectionType”,

“collectionName”: “articles”,

“info”: {

"singularName": "article",

"pluralName": "articles",

"displayName": "Article",

"description": ""

},

“options”: {

"draftAndPublish": true

},

“pluginOptions”: {},

“attributes”: {

"Title": {

  "type": "string",

  "required": true

},

"Content": {

  "type": "richtext",

  "required": true

},

"Image": {

  "type": "media",

  "multiple": false,

  "required": false,

  "allowedTypes": [

    "images",

    "files",

    "videos",

    "audios"

  ]

},

"category": {

  "type": "relation",

  "relation": "manyToOne",

  "target": "api::category.category",

  "inversedBy": "articles"

},

"Description": {

  "type": "string",

  "required": true

},

"tags": {

  "type": "relation",

  "relation": "manyToMany",

  "target": "api::tag.tag",

  "inversedBy": "articles"

},

"admin_user": {

  "type": "relation",

  "relation": "oneToOne",

  "target": "admin::user"

}

}

}