Column of type UUID fails to insert default

[hectorcanto]

Hi everyone,
With these versions:
Eve==0.7.10, Eve-SQLAlchemy==0.7.0, Cerberus==0.9.2, Flask-SQLAlchemy==2.3.2, SQLAlchemy==1.2.12, python 3.6.5, and postgres 9.5.5
I'm trying to insert a default value in a UUID primary key column defined as follows:

from sqlalchemy.dialects.postgresql import UUID
id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)

I added a custom serializer:

from eve.io.base import BaseJSONEncoder
class CustomJSONEncoder(BaseJSONEncoder):
    def default(self, obj):
        if isinstance(obj, UUID):
            return str(obj)
        elif isinstance(obj, datetime):
            return obj.timestamp()

If I insert the row manually (db.session.add()) and make a GET, it works.

But when serializing the response of a POST, it fails.
The JSON serializer receives a function instead of an object.

Digging with the debugger I found that the json encoder holds this two lists:

o = SON[('id', <function uuid4 at 0x7f26fdd29158>)}
chunks = [id": "e95859ca-22dd-440f-b442-c076b8a33dc9"]

This seems incongruent.
If I go up in the callback chain until when the resource appears, I already have the id as a function, the same happens with another datetime field, while not with created_at, or updated_at (custom LAST_UPDATE).

I also tried to add a UUID Validator as suggested here:
http://docs.python-eve.org/en/latest/tutorials/custom_idfields.html#uuid-validation
and changing the schema type from string to uuid:

_DOMAIN["tasks"]["schema"]["id"]["type"] = "uuid"

Am I missing something regarding the validator or the schema?

As a workaround, I've tried to use server_default=sqlalchemy.text("uuid_generatev4()"), after installing the extension uuid-ossp, and it worked, but I prefer to have the generation on the language.

[ernests]

I got it working.

Here is my approach:

from sqlalchemy import (
    Boolean,
    Column,
    DateTime,
    ForeignKey,
    Integer,
    String,
    Text
)

import sqlalchemy.dialects.postgresql as postgresql
import uuid
from sqlalchemy.sql import func

Base = declarative_base()

class Column(Base):
    _id = Column(postgresql.UUID(as_uuid=True),
        primary_key=True,
        default=uuid.uuid4,
        unique=True
    )
    _created = Column(DateTime,
                      default=func.now())
    _updated = Column(DateTime,
                      default=func.now(),
                      onupdate=func.now())
    _etag = Column(Text)

I had to set type of _id for each column to be "uuid - DOMAIN["tasks"]["schema"]["_id"]["type"] = "uuid"

Implemented UUIDValidator and UUIDEncoder from here - https://docs.python-eve.org/en/latest/tutorials/custom_idfields.html#custom-jsonencoder

and passed it to Eve app:

app = Eve(validator= UUIDValidator,
              json_encoder= UUIDEncoder,
              data=SQL)

One key aspect is that you have to generate _id yourself - uuid.uuid4().hex - and pass it as part of POST. It is not needed when you add straight to db.

Hope it helps someone :)