SQLAlchemy: Add support for 'autogenerate_uuid' in column creation

[surister]

Solves crate/sqlalchemy-cratedb#102

Where by setting the column:

Column('x', sa.Text, primary_key=True, crate_autogenerate_uuid=True)

will generate:

x STRING DEFAULT gen_random_text_uuid() NOT NULL

Checklist

• [x] CLA is signed
• [x] Docs
• [x] Tests

[surister]

Circling back to this, I'm effectively checking against

Column(..., server_default='value')

with

if default is not None:
    raise sa.exc.CompileError(
            "Can only have one default value but server_default"
            " and crate_generate_uuid are set"
               )

But It'd probably also clash with Column(..., default=lambda:str(uuid.uuid4()))

I will have a deeper look at this.

[surister]

Hi @surister,

thanks for your patch. It looks good to me so far.

Regarding your question about option clashes: I don't know if and how SQLAlchemy prevents to use both default and server_default, or if it doesn't do at all.

Depending on how SQLAlchemy handles that, we should not necessarily add anything on top - only if it makes sense, and doesn't overcomplicate the code.

With kind regards, Andreas.

Using default + crate_autogenerate_uuid doesn't really 'clash', it seems that CrateDB just ignores the DEFAULT directive and puts in whatever value it gets on the INSERT

[surister]

More importantly when using it as:

Column(..., crate_autogenerate_uuid=True, primary_key=True)

We get the following warning:

SAWarning: Column 'aaaa3.x' is marked as a member of the primary key for table 'aaaa3', but has no Python-side or server-side default generator indicated, nor does it indicate 'autoincrement=True' or 'nullable=True', and no explicit value is passed.  Primary key columns typically may not store NULL.

Because technically, there is no autoincrement_column and server_default nor default is set, so SQLAlchemy cannot assure that the value will not be Null (and nullable in this case is set to False by default)

SQLAlchemy warning source

[amotl]

Dear @surister,

So, the outcome for this topic could be essentially to cover that detail about func.gen_random_text_uuid() within the documentation at, for example, docs/sqlalchemy.rst.

I've just submitted crate/crate-python#585, so I am closing this. Apologies again that I was giving wrong guidance on this matter through crate/sqlalchemy-cratedb#102.

With kind regards, Andreas.