[Developers] querying a property in reverse

[Augusto Callejas]

hi-

some properties do not have an inverse, like "Country of origin" for the
type "/film/film":

  http://mjtemplate.org/examples/schemas/type.html?id=/film/film

so its easy to query the country for a given film in one direction:

{
  "query":[{
    "country":{
      "name":null,
      "type":"/location/country"
    },
    "name":"The Matrix",
    "type":"/film/film"
  }]
}

but not in the other direction:

{
  "query":[{
    "/type/reflect/any_reverse":[{
      "link":{
        "master_property":"/film/film/country"
      },
      "name":"The Matrix",
      "type":"/film/film"
    }],
    "name":null,
    "type":"/location/country"
  }]
}

this problem arises when you have a general tree as a query and you need to
traverse in a certain direction.  the simplest example is a query of the
form:

TypeA-->TypeB<--TypeC

where the directed arrow is a master property but does not have a reverse
property.  any choice of the start of the query results in having to specify
the reverse property of a master property that doesn't have one.

augusto.

On 8/28/07 9:22 PM, "John Giannandrea" <jg at metaweb.com> wrote:

> 
> Augusto Callejas wrote:
>> however, say i want to always want to start from the performance,
>> but i don't know the relationship from performance to film is
>> "film" but i
>> know the relationship from film to performance is "starring".  how
>> can i
>> query on the "starring" property but in reverse without having to
>> perform a
>> separate query for the reverse property?
> 
> You can do this in a roundabout way, with something like this:
> 
> {
>    "query":[{
>      "/type/reflect/any_master":[{
>        "link":{
>          "master_property":{
>            "reverse_property":"/film/film/starring"
>          }
>        },
>        "name":"The Star Wars Holiday Special",
>        "type":"/film/film"
>      }],
>      "character":"Han Solo",
>      "type":"/film/performance"
>    }]
> }
> 
> However this requires that you know which property is the master and
> reverse
> otherwise you would need two clauses mirroring each other.
> 
> Im curious about your use case that would know one property id but
> not be able to find the inverse from the schema.   Can you say a
> little more
> about how this situation would arise in your app?
> 
> -jg
>