Context

To correctly get the instances which are available for an attribute of a transition which is a reference, the query context must be defined.

Example: to find out which entites (attribute "entity") a user may set as the base entity for a new saved query (transition "add" on "extensions.query"), this will find the answer:

POST #/data/system/entity/query

{
  "context": { 
    "module" : "extensions", 
    "entity" : "query", 
    "transition" : "add", 
    "attribute" : "entity"
  }
}

If, in this list, the instance with id 5 is listed, this will be a valid request:

POST /data/extensions/query/transitions/add

{ 
  "entity" : 5,
  #other parameters
  
}

This is necessary because

depending on the workflow, some references may not be available
the user may not have access to all references

Remember that this is just one of the options a query takes, it can still be combined with all the others (page, conds, attrs etc).

{
  "context": {
    "module": "core",
    "entity": "unit",
    "transition": "site_admin_create",
    "attribute": "parent"
  },
  "conds": {
    "name": {
      "-like": "S%"
    }
  }
}