[Sqlalchemy-tickets] Issue #4168: Nested composite column types (zzzeek/sqlalchemy)

[Psyche N. <iss...@bi...>]

New issue 4168: Nested composite column types
https://bitbucket.org/zzzeek/sqlalchemy/issues/4168/nested-composite-column-types

Psyche NA:

According to the [document](http://docs.sqlalchemy.org/en/latest/orm/composites.html)

I have a `Point` class:


```
#!python

class Point(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __composite_values__(self):
        return self.x, self.y
```

I could create a mapping to a table `vertices`, which represents two points as x1/y1 and x2/y2. 
```
#!python

class Vertex(Base):
    __tablename__ = 'vertices'

    id = Column(Integer, primary_key=True)
    x1 = Column(Integer)
    y1 = Column(Integer)
    x2 = Column(Integer)
    y2 = Column(Integer)

    start = composite(Point, x1, y1)
    end = composite(Point, x2, y2)
```
But the problem is that I have two classes `Point` and `Line`:

```
#!python

class Point(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y


class Line(object):
    def __init__(self, start: Point, end: Point):
        self.start = start
        self.end = end
```
I could not create a mapping to a table `Shape` like this:


```
#!python
class Shape(Base):
    __tablename__ = 'shapes'

    id = Column(Integer, primary_key=True)
    a1 = Column(Integer)
    a2 = Column(Integer)
    b1 = Column(Integer)
    b2 = Column(Integer)
    c1 = Column(Integer)
    c2 = Column(Integer)
    d1 = Column(Integer)
    d2 = Column(Integer)
    ...

    line1 = composite(Line, composite(Point, a1, a2), composite(Point, b1, b2))
    line2 = composite(Line, composite(Point, c1, c2), composite(Point, d1, d2))
    ...
```
Maybe the example above is not good enough. But the point is that could composite be nested?