espocrm-base/application/Espo/Resources/metadata/entityDefs/Subscription.json

{
    "fields": {
        "id": {
            "type": "id",
            "dbType": "bigint",
            "autoincrement": true
        },
        "entity": {
            "type": "linkParent"
        },
        "user": {
            "type": "link"
        }
    },
    "indexes": {
        "userEntity": {
            "columns": ["userId", "entityId", "entityType"]
        }
    }
}