#object relation mapping

Lookup Tables

Lookup tables typically come in two flavors, domain-specific and OTLT.

Domain-specific Lookup Tables

Domain-specific lookup tables describe a single domain and help to maintain proper database normalization and promote data integrity through well defined foreign keys.

database=> select * from proc_status; id | name | description ----+------------------+----------------------- 1 | ALIVE | Process is alive. 2 | STARTING | Process is starting. 3 | STOPPING | Process is stopping. 4 | STOPPED | Process is stopped. 5 | DEAD | Process is dead.

Notice in the table above that our primary key is an integer. On occasion you may come across lookup tables which omits the id column and instead use the name column as the primary key.

While using a string column as the primary key can make interacting with lookup tables easier, it will result in increased database size. The increase in size can be substantial, especially if you have a large number of rows referencing your lookup tables.

Moving forward we’ll operate under the assumption that our lookup tables contain integer primary keys.

One True Lookup Table's

One True Lookup Table's (OTLT) denormalize multiple domains into a single lookup table.

database=> select * from lookup; id | context | name | description ----+------------+------------+-------------------- 1 | STATUS | Alive | Process is alive. 2 | STATUS | Starting | Process is starting. 3 | STATUS | Stopping | Process is stopping. 4 | STATUS | Stopped | Process is stopped. 5 | STATUS | Dead | Process is dead. 6 | COLOR | Red | Red 7 | COLOR | Blue | Blue 8 | COLOR | Green | Green

Proponents of OTLT maintain that combining multiple lookup tables into a single table simplifies the overall database schema and its maintenance.

Unfortunately, the price of this simplification is paid for with data integrity. A single lookup table, means related tables will get stuck with vague foreign keys.

Domain-specific Lookup Tables vs. OTLT

We don’t believe that the simplification gains from OTLT justify the data integrity loss. From this point on, any reference to a lookup table will refer to a domain-specific lookup table, not an OTLT.

O/R Mapping and Lookup Tables

Leveraging O/R mapping classes for lookup tables is cumbersome. Consider the code for accessing the proc_status lookup table (described above) using both Django and SQLAlchemy:

#Django ORM #create Proc model which contains a foreign key, #status, to the ProcStatus model status = ProcStatus.objects.get(name="ALIVE") process = Proc( name='myapp', pid=1234, status=status) process.save() #read all procs procs = Proc.objects.select_related("status").all() #read all ALIVE and DEAD procs statuses = ProcStatus.objects.\ filter(name__in=["ALIVE", "DEAD"]).all() procs = Proc.objects.select_related("status").\ filter(status__in=statuses).all()

Or

#SQLAlchemy ORM session = Session() #create Proc model which contains a foreign key, #status, to the ProcStatus model status = session.query(ProcStatus).\ filter_by(name="ALIVE").one() process = Proc( name='myapp', pid=1234, status=status) session.commit() #read all procs procs = session.query(Proc).\ options(joinedload(Proc.status)).\ all() #read all ALIVE and DEAD procs statuses = session.query(ProcStatus).\ filter(ProcStatus.name.in_(["ALIVE", "DEAD"])\. all() procs = session.query(Proc).\ options(joinedload(Proc.status)).\ filter(Proc.status_id.in_([s.id for s in statuses])).\ all()

The first thing you may have noticed about this code is that both creating and reading the Proc object requires additional database work in order to determine the ProcStatus id's. While true, this isn't my biggest problem with the code.

My biggest issue with the above code is its awkwardness. The first OO abstraction that comes to mind when I think lookup table is Enum, not Class. In other words, I'd like the above code to look more like the following:

#SQLAlchemy ORM session = Session() #create process = Proc( name='myapp', pid=1234, status_id=ProcStatus.ALIVE) session.commit() #read all procs procs = session.query(Proc).all() #read all ALIVE and DEAD procs statuses = [ProcStatus.ALIVE, ProcStatus.DEAD] procs = session.query(Proc).\ filter(Proc.status_id.in_(statuses)).\ all()

ORM Enums for Lookup Tables

ORM Enums for lookup tables look compelling, assuming they meet the following constraints:

Enum-like attribute access, i.e. ProcStatus.ALIVE

Enum values keep in sync with the lookup table - no hard coding

Database access is safely throttled

Enums should not require instantiation

Below is the full source code for our Enum class. Take a quick look to get a feel for the overall structure. Don't worry about fully understanding the code, we'll break it down in the next sections.

class DBEnumMeta(type): def __new__(cls, name, bases, attributes): #create new class new_class = super(DBEnumMeta, cls).__new__( cls, name, bases, {}) new_class.load_timestamp = 0 new_class.KEYS_TO_VALUES = {} new_class.VALUES_TO_KEYS = {} if not attributes.get("base", False): required = [ "model_class", "key_column", "value_column", "db_session_factory" ] for attribute in required: if attribute not in attributes: msg = "'%s' att required" % attribute raise RuntimeError(msg) #Add class attributes to new class for name, value in attributes.items(): new_class.add_to_class(name, value) try: if "base" not in attributes: new_class.load() except Exception as error: logging.exception(error) return new_class def __getattr__(cls, attribute): if cls.should_load(attribute): try: cls.load() except Exception as error: logging.exception(error) if attribute not in cls.KEYS_TO_VALUES: msg = "no such attribute '%s'" % attribute raise AttributeError(msg) return cls.KEYS_TO_VALUES[attribute] def add_to_class(cls, name, value): if hasattr(value, "contribute_to_class"): value.contribute_to_class(cls, name) else: setattr(cls, name, value) def should_load(cls, attribute): result = False elapsed = time.time() - cls.load_timestamp if attribute in cls.KEYS_TO_VALUES: result = elapsed > cls.expire and \ elapsed > cls.throttle else: result = elapsed > cls.throttle return result def load(cls): try: cls.load_timestamp = time.time() session = None keys_to_values = {} values_to_keys = {} session = cls.db_session_factory() models = session.query(cls.model_class).all() for model in models: key = getattr(model, cls.key_column) value = getattr(model, cls.value_column) keys_to_values[key] = value values_to_keys[value] = key cls.KEYS_TO_VALUES = keys_to_values cls.VALUES_TO_KEYS = values_to_keys session.commit() except Exception: if session: session.rollback() raise finally: if session: session.close() class DBEnum(object): __metaclass__ = DBEnumMeta base = True expire = 3600 throttle = 60 def __init__(*args, **kwargs): raise RuntimeError("Enum should not be instantiated")

Enum-like attribute access

Enum-like attribute access, i.e. ProcStatus.ALIVE , can be implemented by adding a __getattr__ method. __getattr__ is invoked whenever an attribute cannot be found in the usual places.

In the case of ProcStatus.ALIVE\, the ALIVE attribute will not be found in the usual places, so __getattr__ will be invoked accordingly. Ignoring the load logic for now, the code below simply returns the database value for ALIVE from the KEYS_TO_VALUES dict, or, if it cannot be found, raises an AttributeError .

.... def __getattr__(cls, attribute): if cls.should_load(attribute): try: cls.load() except Exception as error: logging.exception(error) if attribute not in cls.KEYS_TO_VALUES: msg = "no such attribute '%s'" % attribute raise AttributeError(msg) return cls.KEYS_TO_VALUES[attribute] ...

Keeping in sync

In order to keep our Enum values in sync with the lookup table we load() the values when the class object is first instantiated.

Additionally, if an Enum property is accessed, which cannot currently be found in the KEYS_TO_VALUES dict, we will reload the data from the lookup table (as long as it does not violate our throttle ).

And for good measure, Enum values can also be set to expire\ . If an Enum property is accessed and its values are expired, the lookup data will be reloaded.

Despite this last measure, it’s important to note that Enum values should never be changed. This feature is intended to load new Enum values on a read operation, not to update existing Enum values that have changed.

The code below leverages SQLAlchemy, but it should be straightforward to modify it to work with Django or your ORM of choice.

... def load(cls): try: cls.load_timestamp = time.time() session = None keys_to_values = {} values_to_keys = {} session = cls.db_session_factory() models = session.query(cls.model_class).all() for model in models: key = getattr(model, cls.key_column) value = getattr(model, cls.value_column) keys_to_values[key] = value values_to_keys[value] = key cls.KEYS_TO_VALUES = keys_to_values cls.VALUES_TO_KEYS = values_to_keys session.commit() except Exception: if session: session.rollback() raise finally: if session: session.close() ...

Throttled database access

It's important that misbehaving code which repeatedly accesses missing Enum values does not result in excessive database requests.

To prevent excessive I/O our Enum class supports a throttle property. When set, the database will be accessed at most once every throttle seconds.

... def should_load(cls, attribute): result = False elapsed = time.time() - cls.load_timestamp if attribute in cls.KEYS_TO_VALUES: result = elapsed > cls.expire and \ elapsed > cls.throttle else: result = elapsed > cls.throttle return result ...

No instantiation

Our Enum class will not require instantiation. This will allow us to use it as follows:

class ProcStatusEnum(DBEnum): model_class = ProcStatus key_column = "name" value_column = "id" db_session_factory = Session expire = 3600 #expire values every 300s throttle = 30 #max 1 db access every 30s print ProcStatusEnum.ALIVE # 1 print ProcStatusEnum.DEAD # 5 print ProcStatusEnum.VALUES_TO_KEYS # { "ALIVE": 1, "STARTING": 2, ... } print ProcStatusEnum.KEYS_TO_VALUES # { 1: "ALIVE", 2: "STARTING", ... }

In order to achieve this behavior, we derive ProcStatusEnum from DBEnum which leverages meta class DBEnumMeta to properly initialize its values when the class object is first created.

class DBEnumMeta(type): def __new__(cls, name, bases, attributes): #create new class new_class = super(DBEnumMeta, cls).__new__( cls, name, bases, {}) new_class.load_timestamp = 0 new_class.KEYS_TO_VALUES = {} new_class.VALUES_TO_KEYS = {} if not attributes.get("base", False): required = [ "model_class", "key_column", "value_column", "db_session_factory" ] for attribute in required: if attribute not in attributes: msg = "'%s' attr required" % attribute raise RuntimeError(msg) #Add class attributes to new class for name, value in attributes.items(): new_class.add_to_class(name, value) try: if "base" not in attributes: #load data from lookup table new_class.load() except Exception as error: logging.exception(error) return new_class ... class DBEnum(object): __metaclass__ = DBEnumMeta base = True expire = 3600 throttle = 60 def __init__(*args, **kwargs): raise RuntimeError("DBEnum should not be instantiated")

Putting it all together

Let's now revisit our lookup table code from earlier. But this time let's leverage our Enum class.

#SQLAlchemy ORM session = Session() #Process status enum for our lookup table class ProcStatusEnum(DBEnum): model_class = ProcStatus key_column = "name" value_column = "id" db_session_factory = Session expire = 3600 #expire values every 300s throttle = 30 #max 1 db access every 30s #create Proc model which contains a foreign key, #status, to the ProcStatus model process = Proc( name='myapp', pid=1234, status_id=ProcStatusEnum.ALIVE) session.commit() #read all procs procs = session.query(Proc).all() #read all ALIVE and DEAD procs statuses = [ProcStatusEnum.ALIVE, ProcStatusEnum.DEAD] procs = session.query(Proc).\ filter(Proc.status_id.in_(statuses)).\ all()

Overall, the code above is much cleaner and better suited for an object oriented environment.