class ModelA(models.Model):
   pointid = models.CharField(max_length=25)
   name = models.CharField(max_length=200)
   geom - gismodels.PointField()
   object = gismodels.GeoManager()

class ModelB(models.Model):
   name = models.CharField(max_length=200)
   line_geom = gismodels.LineStringField(null=True)
   color = RGBColorField(null=True)
   object = gismodels.GeoManager()

>>> import psycopg2
>>> conn = psycopg2.connect(dbname=..., port=..., user=...,
                            password=..., host=...)

>>> cur.execute("SELECT (???) (ST_MakeLine(ARRAY[ST_MakePoint(???)]);")
>>> cur.fetchall()

class ParentModel():
    name = models.CharField()
    geom = models.LinestringField()

class ChildModel():
    parent = models.ForeignKey(ParentModel)
    order= models.IntegerField()
    geom = models.PointField()

childs = parent.childmodel_set.all()

childs = parent.childmodel_set.all().orderby('order')

# there are many ways to achieve this. you can use a for, for example
# or a list comprehension, shown below
linestring = LineString([child.geom for child in childs])
parent.geom = linestring
parent.save()