Relational Databases: Object Relational

Mappers - SQLObject

BCHB5242014

Lecture 22

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Outline

Object relational mappers Tables as classes, rows as instances Advantages & disadvantages

Minimal SQLObject example Legacy databases Exercises

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Relational Databases

Store information in a table Rows represent items Columns represent items' properties or

attributes

Name Continent RegionSurface

AreaPopulatio

nGNP

BrazilSouth

America South America 854740317011500

0 776739

Indonesia Asia Southeast Asia 190456921210700

0 84982

India AsiaSouthern and Central

Asia 328726310136620

00 447114

China Asia Eastern Asia 957290012775580

00 982268

Pakistan AsiaSouthern and Central

Asia 79609515648300

0 61289

United States

North America North America 9363520

278357000 8510700

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... as Objects

Objects have data members or attributes.

Store objects in a list oriterable.

Abstract awaydetails of underlyingRDBMS

c1 = Country()c1.name = 'Brazil'c1.continent = 'South America'c1.region = 'South America'c1.surfaceArea = 8547403c1.population = 170115000c1.gnp = 776739

# initialize c2, ..., c6

countryTable = [ c1, c2, c3, c4, c5, c6 ]

for cnty in countryTable:    if cnty.population > 100000000:        print cnty.name, cnty.population

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Pros and Cons

Pros: Learn one language Ease of development Simplified joins One set of data-types Easy storage of higher-level

objects Can apply the power of

python as necessary Abstract away RDBMS Distribute CPU load

Cons: Execution speed Sometimes forced into poor

strategies Optimal SQL construct may

be impossible Tend not to take advantage

of RDBMS quirks. Can be difficult to apply to

legacy databases

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SQLObject

Just one of many object-relational mappers Each tool makes different tradeoffs in

Table/row/attribute abstraction How much SQL you need to know Overhead Ease of adapting to legacy databases

SQLObject is almost completely devoid of SQL and is almost entirely "objecty".

See http://sqlobject.org.

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Minimal SQLObject Example:Define the database model (model.py)

from sqlobject import *import os.path

dbfile = 'myworld.db3'

# Magic formatting for database URIconn_str = os.path.abspath(dbfile)conn_str = 'sqlite:'+ conn_str

sqlhub.processConnection = connectionForURI(conn_str)

class Country(SQLObject):    name = StringCol()    continent = StringCol()    region = StringCol()    surfaceArea = FloatCol()    population = IntCol()    gnp = FloatCol()

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Minimal SQLObject Example: Populate the database

from model import Country

# Initialize the tableCountry.createTable()

# Add some rowsc = Country(name="Brazil", continent="South America",            region="South America", surfaceArea=8547403,            population=170115000, gnp=776739)c = Country(name="China", continent="Asia",            region="Eastern Asia", surfaceArea=9572900,            population=1277558000, gnp=982268)# ...c = Country(name="United States", continent="North America",            region="North America", surfaceArea=9363520,            population=278357000, gnp=8510700)

# Retrieve and print all countriesfor c in Country.select():    print c.id, c.name, c.continent, c.gnp

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Minimal SQLObject Example: Access/Change the database

from model import Country

# Change country #6c = Country.get(6)c.name = 'United States of America'

# Retrieve and print all countriesfor c in Country.select():    print c.id, c.name, c.continent, c.gnp

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Minimal SQLObject Example:Access the rows as objects

from model import Country

# Select countries with more than 500,000,000 in populationfor c in Country.select(Country.q.population >= 500000000):    print "A:", c.id, c.name, c.population

# Select countries that start with 'U'for c in Country.select(Country.q.name.startswith("U")):    print "B:", c.id, c.name, c.population

# Lookup by id, exactly 1 country with each idc = Country.get(5)print "C:", c.id, c.name, c.population

# Get exception for bad id# c = Country.get(100)

# Shortcut for select, countries with continent == 'Asia'for c in Country.selectBy(continent = 'Asia'):    print "D:", c.id, c.name, c.population

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Legacy databases

If the legacy database is well-structured, SQLObject can figure out (most of) the definitions

If there is no id column... Need to tell SQLObject what to use for the ID. May need to specify the id at instantiation time.

Have to fill in MultipleJoins and ForeignKeys yourself Need to declare which columns in two different tables

should correspond. Enables SQLObject to make relationships explicit Enables SQLObject to turn joins into lists

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Legacy databasesfrom sqlobject import *import os.path

dbfile = 'taxa.db3'

conn_str = os.path.abspath(dbfile)conn_str = 'sqlite:'+ conn_str

sqlhub.processConnection = connectionForURI(conn_str)

class Taxonomy(SQLObject):    class sqlmeta:        idName = "tax_id"        fromDatabase = True    names = MultipleJoin('Name', joinColumn="tax_id")

class Name(SQLObject):    class sqlmeta:        fromDatabase = True    taxa = ForeignKey('Taxonomy', dbName="tax_id")

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Legacy databases# Set up data-modelfrom model import *

# get homo sapienshs1 = Taxonomy.get(9606)

# select the Taxonomy object# with scientific name Homo sapienshs2 = Taxonomy.selectBy(scientificName='Homo sapiens')[0]

# get the name humantry:    hsname = Name.selectBy(name='human')[0]except IndexError:    print "Can't find name 'human'"    sys.exit(1)

# get the Taxonomy object from the Name object# Uses the magic Foreign Key attributehs3 = hsname.taxa

# hs1, hs2, hs3 the same!print hs1print hs2print hs3

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Legacy databases# Set up data-modelfrom model import *

# get homo sapienshs = Taxonomy.get(9606)

# find rows in the Name table with taxa the same as hs# Use ForeignKey to create condition, equality test# between objectscondition = (Name.q.taxa == hs)for n in Name.select(condition):    print n

# find rows in the Name table corresonding to hs# Easy shortcut, using MultipleJoin iterablefor n in hs.names:    print n.name, "|", n.nameClass

# More general conditionscondition = Name.q.name.startswith('Da')for n in Name.select(condition):    print n.name, "|", n.nameClass

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Exercises

Read through the SQLObject documentation

Write a python program using SQLObject to lookup the scientific name for a user-supplied organism name.