Reflect a PostgreSQL view in Python’s SQLAlchemy

I ran into this problem a while ago, where I had a simple view created by joining two tables with a 1:1 relationship in PostgreSQL but SQLAlchemy didn’t like my view.

Background

The core problem comes from SQLAlchemy being an ORM (object relationship mapper) made to fit most databases moderately well for most common use cases. However sometimes it does not fit their model as well. One such case is views and in particular views in PostgreSQL.

Why’s that? you might ask. Well here’s the thing, to the best of my knowledge there’s as of this date no way to natively reflect a view in SQLAlchemy. Instead one must use the Table() construct to initiate the view, and herein lies the foundational problem, see table’s in SQLAlchemy requires a primary key to use as a hash while views in PostgreSQL can’t. Clearly a point of conflict.

A problematic domain model — sample code attached

I’ve oversimplified the model and view to press on the actual issue and not steer focus towards unrelated code. In my case we’ve got a base model for Fields followed by a couple of related models for specialised instances of these fields. One such is Template_Field shown below, this is a mere extension of the common field type and is never accessed directly but rather through the view named Template_Field_View.

Field.psql

CREATE TABLE IF NOT EXISTS quick_bits.Field (
id serial PRIMARY KEY,
order_Index INTEGER,
label TEXT,
key VARCHAR(255)
);

Template_Field.psql
Templates in them self are not interesting for this obstacle and are thus left out, however I left the relation in Template_Field for illustrative purposes.

CREATE TABLE IF NOT EXISTS quick_bits.Template_Field(
template integer REFERENCES quick_bits.Template(id),
field integer PRIMARY KEY REFERENCES quick_bits.Field(id),
);

Template_Field_View.psql

CREATE OR REPLACE VIEW quick_bits.template_field_view AS
SELECT
field.id,
template_field.template,
field.label,
field.key,
field.order_index,
FROM quick_bits.template_field as template_field
INNER JOIN quick_bits.field as field
ON field.id = template_field.field
;

Problems start surfacing

The most naïve approach for me were to just try reflecting the view straight away as a table with SQLAlchemy in my python code.

# ...

def init_table(name):
return Table(name, meta, autoload=True, schema=config.DB_SCHEMA)

class Field(Base):
__table__ = init_table('field')

class TemplateField(Base):
__table__ = init_table('template_field')

class TemplateFieldView(Base):
__table__ = init_table('template_field_view')

Now let’s see what happens if we try to read data using this model.

class TemplateFieldView(Base):
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/api.py", line 64, in __init__
_as_declarative(cls, classname, cls.__dict__)
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 88, in _as_declarative
_MapperConfig.setup_mapping(cls, classname, dict_)
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 103, in setup_mapping
cfg_cls(cls_, classname, dict_)
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 135, in __init__
self._early_mapping()
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 138, in _early_mapping
self.map()
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 534, in map
**self.mapper_args
File "<string>", line 2, in mapper
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 677, in __init__
self._configure_pks()
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 1277, in _configure_pks
(self, self.mapped_table.description))
sqlalchemy.exc.ArgumentError: Mapper Mapper|TemplateFieldView|template_field_view could not assemble any primary key columns for mapped table 'template_field_view'

The following error does seem daunting but we can quickly see that the problem lies in mapping a view without primary keys as a table which requires said key.

First workaround attempt

As we already got a field id which is unique we could try forcing SQLAlchemy into thinking it’s a primary key.

My first approach were something along these lines.

class TemplateFieldView(Base):
__table__ = init_table('template_field_view')
id = Column(Integer, primary_key=True)

Let’s try it out.

File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 131, in __init__
self._setup_table()
File "/Users/hultner/Development/quick_bit_test/venv/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 403, in _setup_table
"specifying __table__" % c.key
sqlalchemy.exc.ArgumentError: Can't add additional column 'id' when specifying __table__

Some progress but no cigar. We can now see that the problem is no longer the primary key but rather that we can’t specify new columns like this in a reflected tables. This is because of the inner workings of SQLAlchemy have already decided on the table structure and doesn’t want to mutate this representation.

The solution

So how do we circumvent this dilemma, after some searching I found the __mapper_args__ property. We can use this to change the behaviour of columns in the table/view. Read more at SQLAlchemy Docs.

So now I have the following code.

class TemplateFieldView(Base):
__table__ = init_table('template_field_view')
__mapper_args__ = {
'primary_key':[__table__.c.id]
}

So let’s try it out.

>>> get_template_field_from_view(54)
Field(id='54', template='3', order_index='1', label='Name', key='name_data', type='FIRST_NAME')>

Success!

There you have it. If anyone got a nicer solution I would love to hear it, until then I hope this is helpful to you!


Originally published at hultner.github.io on October 23, 2017.