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138 lines
4.5 KiB
Python
138 lines
4.5 KiB
Python
"""
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This example/recipe has been contributed by Valentino Volonghi (dialtone)
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Mapping arbitrary objects to a PostgreSQL database with psycopg2
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- Problem
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You need to store arbitrary objects in a PostgreSQL database without being
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intrusive for your classes (don't want inheritance from an 'Item' or
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'Persistent' object).
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- Solution
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"""
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from datetime import datetime
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import psycopg2
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from psycopg2.extensions import adapt, register_adapter
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# Here is the adapter for every object that we may ever need to
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# insert in the database. It receives the original object and does
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# its job on that instance
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class ObjectMapper(object):
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def __init__(self, orig, curs=None):
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self.orig = orig
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self.tmp = {}
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self.items, self.fields = self._gatherState()
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def _gatherState(self):
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adaptee_name = self.orig.__class__.__name__
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fields = sorted([(field, getattr(self.orig, field))
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for field in persistent_fields[adaptee_name]])
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items = []
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for item, value in fields:
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items.append(item)
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return items, fields
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def getTableName(self):
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return self.orig.__class__.__name__
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def getMappedValues(self):
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tmp = []
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for i in self.items:
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tmp.append("%%(%s)s"%i)
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return ", ".join(tmp)
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def getValuesDict(self):
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return dict(self.fields)
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def getFields(self):
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return self.items
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def generateInsert(self):
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qry = "INSERT INTO"
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qry += " " + self.getTableName() + " ("
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qry += ", ".join(self.getFields()) + ") VALUES ("
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qry += self.getMappedValues() + ")"
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return qry, self.getValuesDict()
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# Here are the objects
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class Album(object):
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id = 0
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def __init__(self):
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self.creation_time = datetime.now()
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self.album_id = self.id
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Album.id = Album.id + 1
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self.binary_data = buffer('12312312312121')
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class Order(object):
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id = 0
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def __init__(self):
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self.items = ['rice','chocolate']
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self.price = 34
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self.order_id = self.id
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Order.id = Order.id + 1
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register_adapter(Album, ObjectMapper)
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register_adapter(Order, ObjectMapper)
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# Describe what is needed to save on each object
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# This is actually just configuration, you can use xml with a parser if you
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# like to have plenty of wasted CPU cycles ;P.
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persistent_fields = {'Album': ['album_id', 'creation_time', 'binary_data'],
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'Order': ['order_id', 'items', 'price']
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}
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print adapt(Album()).generateInsert()
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print adapt(Album()).generateInsert()
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print adapt(Album()).generateInsert()
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print adapt(Order()).generateInsert()
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print adapt(Order()).generateInsert()
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print adapt(Order()).generateInsert()
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"""
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- Discussion
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Psycopg 2 has a great new feature: adaptation. The big thing about
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adaptation is that it enables the programmer to glue most of the
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code out there without many difficulties.
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This recipe tries to focus attention on a way to generate SQL queries to
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insert completely new objects inside a database. As you can see objects do
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not know anything about the code that is handling them. We specify all the
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fields that we need for each object through the persistent_fields dict.
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The most important lines of this recipe are:
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register_adapter(Album, ObjectMapper)
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register_adapter(Order, ObjectMapper)
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In these lines we notify the system that when we call adapt with an Album instance
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as an argument we want it to istantiate ObjectMapper passing the Album instance
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as argument (self.orig in the ObjectMapper class).
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The output is something like this (for each call to generateInsert):
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('INSERT INTO Album (album_id, binary_data, creation_time) VALUES
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(%(album_id)s, %(binary_data)s, %(creation_time)s)',
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{'binary_data': <read-only buffer for 0x402de070, ...>,
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'creation_time': datetime.datetime(2004, 9, 10, 20, 48, 29, 633728),
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'album_id': 1}
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)
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This is a tuple of {SQL_QUERY, FILLING_DICT}, and all the quoting/converting
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stuff (from python's datetime to postgres s and from python's buffer to
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postgres' blob) is handled with the same adaptation process hunder the hood
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by psycopg2.
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At last, just notice that ObjectMapper is working for both Album and Order
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instances without any glitches at all, and both classes could have easily been
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coming from closed source libraries or C coded ones (which are not easily
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modified), whereas a common pattern in todays ORMs or OODBs is to provide
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a basic 'Persistent' object that already knows how to store itself in the
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database.
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"""
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