Re: Context lenses to set/get values in json values. - Mailing list pgsql-hackers

On 10/08/2014 04:38 AM, Paweł Cesar Sanjuan Szklarz wrote:

Hello.

I am interested in the json type on postgresql. I would like to implement additional operations on the json structure that may extract/insert table like information from the json tree structure.
I have a implementation on javascript that shows this type of operations. You can see examples in this page
https://github.com/paweld2/eelnss/wiki

Following the examples in the previous page, it may by possible to implement a function similar to json_populate_record to extract multiple records from a single json value, for example:
select * from json_populate_records_with_clen(null::myrowtype_users, 'app.users.{:uID}.(email,data.name <http://data.name>,isActive)', '... nested json value ...')

may return
uID  |  email                          | name              | isActive
--------------------------------------------------------------------------
"u1" | "admin@pmsoft.eu <mailto:admin@pmsoft.eu>"    | "administrator" | true
"u2" | "normal@pmsoft.eu <mailto:normal@pmsoft.eu>"   | "user"               | true
"u3" | "testUser@pmsoft.eu <mailto:testUser@pmsoft.eu>" | "testUser"        | false


Also, assuming that we have a table User as above (uID, email, name, isActive), with context lenses it is very simple to map the table to a json object. I assume that a similar api to table_to_xml,query_to_xml may be provided:

table_to_json( Person, 'app.users.{:uID}.(email,data.name <http://data.name>,isActive)');
query_to_json( 'select * from Person where ... ', 'app.users.{:uID}.(email,data.name <http://data.name>,isActive)');


I don't know the details about the integration of functions/operators to sql queries, but because context lenses maps between tables and tree objects, it may be possible to use a column json value as a separate table in the queries. Assume the table
create table Person {
  pID  Integer
  address Json
}
then it  may be possible to query:
select * from Person as P left join ( select * from json_populate_records_with_clen(null::addressType, 'addres.(street.number, street.local,city.code,city.name <http://city.name>)', P.address);

A final api for such functions needs to be defined. If such functions may be usefull, I can try to prepare a implementation in postgres base code.

I don't think we need to import Mongo type notation here. But there is probably a good case for some functions like:

   json_table_agg(anyrecord) -> json

which would work like json_agg() but would return an array of arrays instead of an array of objects. The caller would be assumed to know which field is which in the array. That should take care of both the table_to_json and query_to_json suggestions above.

In the other direction, we could have something like:

    json_populate_recordset_from_table(base anyrecord, fields text[], jsontable json) -> setof record

where jsontable is an array of arrays of values  and fields is a corresponding array of field names.

I'm not sure how mainstream any of this is. Maybe an extension would be more appropriate?

cheers

andrew