Hi neutrinos,

In Neutron many attributes are stored in database fields. The size of these
fields therefore determines the maximum length of the attribute values.

I would like to get some consistency in place around how we define the
constants and where they are used. Here are my thoughts...

1. Raw sizes in alembic migrations

In the alembic migrations which build the DB schema, we should use the raw
number value of the field size.

2. FOO_FIELD_SIZE in the sqlalchemy models

In the sqlalchemy models, we should use the <field>_FIELD_SIZE constants
defined in neutron_lib/db/constants.py

3. Everywhere else, use FOO_FIELD_SIZE, or another constant (like FOO_MAX_LEN)
based on FOO_FIELD_SIZE.

"Why raw numbers in alembic migrations?", you may ask. Well, we have tests
that verify that the models match the schema generated by migrations. If both
the models and the migrations use the constants then the tests would not
detect if a patch changes the constant value.

By using raw numbers in migrations, together with our rule of not allowing
changes to existing migrations, we allow the tests to detect and fail on any
attempt to alter a FIELD_SIZE constant.

Let me know if this makes sense or if you think it's a terrible idea.

If there are no objections, I intend to submit a patch or patches to:
 - replace constants with numbers in existing migrations
 - ensure all models use the appropriate constants

Existing code uses FOO_MAX_LEN in a lot of places. In most of these places it
would make sense to simply switch to using FOO_FIELD_SIZE. However, some code
may be quite far removed from the DB and would look better by sticking to
FOO_MAX_LEN. I added item 3. above to allow for that.

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