addField ForeignKey during migrations create unusable constraint name

[Gromish]

Env: Django 1.8.11 + Postgis

I'm adding some ForeignKeys on a MyModel. The models pointed are in another schema ("cartography").

makemigrations

no errors

migrate

One error. Can't create the constraint because the generated name. But I'm adding 10 fields, really similar between them. Only one is giving that stupid error. I can't specify the constraint name anywhere.

class myModel(models.Model)
    zps_calculated = models.ForeignKey( Cartography_zps, verbose_name="zps_calcolato", null=True, blank=True, on_delete=models.SET_NULL)
    zsc_sic_sir_calculated = models.ForeignKey( Cartography_zsc_sic_sir, verbose_name="zsc_sic_sir_calcolato", null=True, blank=True, on_delete=models.SET_NULL)
    manyothersdata = "xxx"

That is the slice of code generated from sqlmigrate (to inspect the code the migration generate). As you see the name of the constraint is the error. 1 on 10 fields is giving the error

CREATE INDEX "segnalazioni_f38ba181" ON "segnalazioni" ("zps_calculated_id");
ALTER TABLE "segnalazioni" ADD CONSTRAINT "se_zps_calculated_id_6844dce0603174b2_fk_"cartography"."zps"_id" FOREIGN KEY ("zps_calculated_id") REFERENCES "cartography"."zps" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "segnalazioni_eb52e53f" ON "segnalazioni" ("zsc_sic_sir_calculated_id");
ALTER TABLE "segnalazioni" ADD CONSTRAINT "cc6ce48808e3a5292779a9787d21e5ad" FOREIGN KEY ("zsc_sic_sir_calculated_id") REFERENCES "cartography"."zsc_sic_sir" ("id") DEFERRABLE INITIALLY DEFERRED;

That is the name giving the error: "se_zps_calculated_id_6844dce0603174b2_fk_"cartography"."zps"_id" I think should be something like: "6844dce0603174b2..."

the model NOT giving the error:

class Cartography_zsc_sic_sir(models.Model):
    id = models.AutoField(primary_key=True)
    slug = models.CharField(max_length=40, blank=True, null=True)
    nome = models.CharField(max_length=60, blank=True, null=True)
    the_geom = models.MultiPolygonField(srid=23032, blank=True, null=True )
    objects = models.GeoManager()

    class Meta:
        managed = False
        db_table = '"cartography"."zsc_sic_sir"'
        verbose_name = 'Cartography - zsc_sic_sir'
        verbose_name_plural = 'Cartography - zsc_sic_sir'
        ordering = ["id","slug"]

    def __unicode__(self):
        return self.nome

that is the model giving the error:

class Cartography_zps(models.Model):
    id = models.AutoField(primary_key=True)
    slug = models.CharField(max_length=40, blank=True, null=True)
    the_geom = models.MultiPolygonField(srid=23032, blank=True, null=True )
    objects = models.GeoManager()

    class Meta:
        managed = False
        db_table = '"cartography"."zps"'
        verbose_name = 'Cartography - ZPS'
        verbose_name_plural = 'Cartography - ZPS'
        ordering = ["id","slug"]

    def __unicode__(self):
        return self.slug

Going further I'm investigating in Django code, backwards.

The

 ' %s ADD CONSTRAINT %s FOREIGN KEY (%s) REFERENCES %s (%s)%s;' %

is in /django/db/backends/base/creation.py row 180

using that

qn = self.connection.ops.quote_name

that SHOULD be the %s constraint name value:

qn(truncate_name(r_name, self.connection.ops.max_name_length()))

Anyone have an hint to help me? I'm gonna look what qn does.

[Simon Charette]

Hi Gromish,

From looking at you models `db_table` options it looks like you are using a
work around to use a table defined in a non default schema (cartography).

Django doesn't officially support custom schema yet[1] but there's an ongoing
effort to get basic support merged in Django 1.10[2] that you could review.

In the meantime I suggest you replace your migration's `AddField` of
`ForeignKey` by `SeparateDatabaseAndState`:

Replace

AddField(
    model_name='myModel',
    name='zps_calculated',
    field=models.ForeignKey(to='app.Cartography_zps'),
)

By

AddField(
    model_name='myModel',
    name='zps_calculated_id',
    field=models.IntegerField(db_index=True),
)
SeparateDatabaseAndState(
    [RunSQL("""

        ALTER TABLE "segnalazioni" ADD CONSTRAINT

        "se_zps_calculated_id_6844dce0603174b2_fk_cartography_zps_id"

        FOREIGN KEY ("zps_calculated_id")
        REFERENCES "cartography"."zps" ("id") DEFERRABLE INITIALLY DEFERRED;

    """)],
    [AlterField(
        model_name='myModel',
        name='zps_calculated_id',
        field=models.ForeignKey(to='app.Cartography_zps', name='zps_calculated')
    )],
)

Cheers,
Simon

[1] https://code.djangoproject.com/ticket/6148
[2] https://github.com/django/django/pull/6162