PostgreSQL – Set a Default Value for a JSONB Property

jsonpostgresql

Let's say that I have a jsonb column with documents like:

{
    "lead": "The lead",
    "video": "An URL to the video"
}

In some cases I also need to store the name of a picture that's uploaded to the server by the user in the JSON. Obviously I don't want clashes on the filenames so I wonder if it's possible to use uuid_generate_v4() as default value:

insert into document (title, content) values (
    "The title of the document",
    '{
        "lead": "The lead",
        "video": "An URL to the video",
        "picture": ":?" -- Here I need the value returned by uuid_generate_v4()
    }'
);

Is it possible or should I get the uuid in the application before inserting the data in the database?

Thanks beforehand.

Best Answer

You can resort to row_to_json for things like this.

insert into document (title, content)
select 'The title of the document', row_to_json(n)
from (
  select 'The lead' as lead,
         'An URL to the video' as video,
         uuid_generate_v4() as picture
  ) n

Related Solutions

Postgresql – Using MongoDB and PostgreSQL together

Some thoughts....

Typically one does not want to store pieces of tightly interrelated information in different systems. The chances of things getting out of sync is significant and now instead of one problem on your hands you have two. One thing you can do with Mongo though is use it to pipeline your data in or data out. My preference is to keep everything in PostgreSQL to the extent this is possible. However, I would note that doing so really requires expert knowledge of PostgreSQL programming and is not for shops unwilling to dedicate to using advanced features. I see a somewhat different set of options than you do. Since my preference is not something I see listed I will give it to you.

You can probably separate your metadata into common data, data required for classes, and document data. In this regard you would have a general catalog table with the basic common information plus one table per class. In this table you would have an hstore, json, or xml field which would store the rest of the data along with columns where you are storing data that must be constrained significantly. This would reduce what you need to put in these tables per class, but would allow you to leverage constraints however you like. The three options have different issues and are worth considering separately:

hstore is relatively limited but also used by a lot of people. It isn't extremely new but it only is a key/value store, and is incapable of nested data structures, unlike json and xml.

json is quite new and doesn't really do a lot right now. This doesn't mean you can't do a lot with it, but you aren't going to do a lot out of the box. If you do you can expect to do a significant amount of programming, probably in plv8js or, if you want to stick with older environments, plperlu or plpython. json is better supported in 9.3 though at least in current development snapshots, so when that version is released things will get better.

xml is the best supported of the three, with the most features, and the longest support history. Then again, it is XML.....

However if you do decide to go with Mongo and PostgreSQL together, note that PostgreSQL supports 2 phase commit meaning you can run the write operations, then issue PREPARE TRANSACTION and if this succeeds do your atomic writes in Mongo. If that succeeds you can then COMMIT in PostgreSQL.

PostgreSQL – How to Update and Delete Property from JSONB Column

Question 1
There should be no signicant drawbacks. As the value is converted back to jsonb anyhow I would guess it would be more efficient to keep it that way the whole time.

Question 2
Just replace your function with the following (only the part WHERE key NOT IN ... added):

CREATE FUNCTION jsonb_merge(JSONB, JSONB) 
RETURNS JSONB AS $$
WITH json_union AS (
    SELECT * FROM JSONB_EACH($1)
    UNION ALL
    SELECT * FROM JSONB_EACH($2)
) SELECT JSON_OBJECT_AGG(key, value)::JSONB
     FROM json_union
     WHERE key NOT IN (SELECT key FROM json_union WHERE value ='null');
$$ LANGUAGE SQL;

Best Answer

Related Solutions

Postgresql – Using MongoDB and PostgreSQL together

PostgreSQL – How to Update and Delete Property from JSONB Column

Related Question