object of type 'SQLAlchemyConnectionField' has no len()

Hello,

I am implementing a connection field.

The excerpts from the code are:

```
# Classes:
class Common(graphene_sqlalchemy.SQLAlchemyObjectType):
class Meta:
abstract = True
...

class MyGraphQLModel(Common):
class Meta:
model = MyModel
interfaces = (graphene.relay.Node, )

# Fields:
mymodels = graphene.List(MyGraphQLModel)
def mymodels(p,i):
query = MyGraphQLModel.get_query(i)
return query.all()

connection_mymodels = graphene_sqlalchemy.SQLAlchemyConnectionField(MyGraphQLModel)
```

The first field (`mymodels`) which is an array without a connection works fine.
The second field which is a connection gives error:

```
"message": "object of type 'SQLAlchemyConnectionField' has no len()",
```

Any ideas? Thanks.

Start with the SQLAlchemyConnectionField implementation and the traceback for the reported len() error, then reproduce it using the MyGraphQLModel schema and connection_mymodels field shown here. Done means the connection field resolves without raising the TypeError and the existing list field remains working.

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