SQL Alchemy Advanced Python Nbn 2007

8/6/2019 SQL Alchemy Advanced Python Nbn 2007

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SQLAlchemy
http://goforward/http://find/http://goback/


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Just like SQLObject

Third party library Object relational manager

Define database tables, connect (map)classes to tables

Table rows are objects / instances What you do to an object, you do to a row

Phrase queries in a morepythonic(less SQL-ish) way

Hide database differences

The database disappears ...


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Making an engine

Connecting is slightly more complicated than SQLObject:

Use a connection string of the formdriver://username:password@host:port/database

Make adatabase enginewith create_engine

encoding and convert_unicode can be used to convertcharacters for all transactions

echo can be set to print raw SQL

Use to make special metadata object

from sqlalchemy import *

conn_str = "mysql://dbuser:dbpass@localhost:8028/epi"

engine = create_engine (conn_str, echo=True)

metadata = MetaData (engine)


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Making a connection

Call connect from engine Connection can execute raw SQL, like DB-API cursor

objects.

Dont forget to close() it!

Actually we dont need to use the connection ...

connection = engine.connect()

result = connection.execute ("SELECT * FROM reports")

# returns a sequence of dictionaries

for row in result:print row[id], row[country]

connection.close()


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Define tables

Instantiating a Table object (not defining a class), defines thecorresponding table.

The general form is Table (, , []+)

Metadata binds table to database

SampleTable = Table (sample,metadata,

Column (ident, String(32), primary_key=True),

Column (locn_lat, Float()),

Column (locn_lon, Float()),

Column (locn_method, String(32), default=EXACT),Column (country, String(64), nullable=False),

Column (date_collected, Date()),

)

SampleTable.create()


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Table options

Defining table is a call.

Create table via .create().

Can create all bound tables with .create all(). Links to actual database via metadata.

Can take autoload parameter to work out columns fromdatabase.


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The awkward way of using the database

Require a call on the table then an explicit execution

query = SampleTable.insert()

query.execute (ident=X47, locn_lat=34.1,

locn_lon=-12.1)

# ... or ...

query = SampleTable.insert().execute (

ident=X47, locn_lat=34.1, locn_lon=-12.1)


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Columns

Oddly familiar:String (length) A string. If length isnt given, the field

width is variable (Text)

UnicodeCol A per StringCol, but with unicode strings.

DateTime Also see Date and Time.Boolean May be implemented as a variety of types.

Integer Duh.

Numeric (precision, length) Precise float with the given overall

width and number of figures after the point.FloatCol (precision) A float, with a level of precision.


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Column parameters

primarykey False Is it a unique id for the row? You can have asmany as you want ...

nullable True Value for this column. when created. Can becallable. If no default, must be provided (like NOT

NULL).default Default value for column. Will call if callable.

Opposite to SQLObject.

oninsert Default value when entry created and no valuesupplied.

onupdate Default value when entry updated and no valuesupplied.


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Column example

A timestamped table for samples, where a source must beprovided, and a user-provided string serves as an id

TimedTable = Table (sample,

metadata,

Column (ident, String(10), primarykey=True )Column (source, String(30), nullable=False)

Column (time_created, DateTime(), de-

fault=func.now()),

Column(time_updated, DateTime(), onup-

date=func.now()),)
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Autoload tables

Derive columns from database

users = Table(users, metadata, autoload=True)

emails = Table(emails, metadata, autoload=True)


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func

An answer to delayed execution / calling of functions: it binds afunction and can later call it with passed parameters.

func.now () Wrapsnow() for later calling.func.max (mylist) Wraps max for later calling max (mylist).

func.substr (users.c.name, 2, 1) Wraps for later callusers.c.name.substr (2, 1)


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Mappers

Objects (in Python) dont automatically map to database tablesand rows. Instead they are mapped.

users = Table (user_table, ...)

class User (object):

pass

usermapper = mapper (User, users)


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Mapped classes

Any mapped classes get the columns of the table as membersfor free.

Mapped classes must be derived from object.

mapper (, )

You can use __init__ and subclass mapped objects Interaction takes place in the context of a session

class Report (object):

def init (self): # ...

def setLocn (self, lat, lon): self.lat =lat self.lon = lon


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Sessions

Sessions are devices for caching and saving the changes in objectsin tables. bind_to parameter is optional. List the objects to besaved.

sess = create_session (bind_to=engine)# ... much later ...

sess.save (changed_objs)

# saves everything

sess.flush()


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select

select method of table searches rows

query = .select ([.q.field ==

]+)

query.execute()

select_by()

>>> query = report_table.select ()

>>> str (query)

SELECT * FROM report ...>>> results = query.execute()


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Results

Query results are sequences of dictionary-like objects Results arelazyand can be sliced

Use fetch syntax

Can (should) close results

>>> results = query.execute()>>> first_row = results.fetchone()

>>> first_row[id]

X47

>>> first_row.id

X47

>>> second_row = results[1]

>>> results.close()

S


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Select options

Use special c attribute to express clauses and conditions Can specify columns to return in select

Can test fields

some_cols_query = report_table.select ([re-

port_table.c.id,

report_table.c.country])

country_query = report_table.select (re-

port_table.c.country==Burma)

next_query = report_table.select (re-port_table.c.country.in_ (

North Korea, South Korea))

l


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select operators

==, >, >=, etc.

.like(), .startswith(), .endswith()

.between(), and .in_()

not_(), and_() and or_() (or ~, &, and |)

+, -, *, /

M l i l l


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Multiple selectors

Use and_, op_ & not to combine conditions.

query = report_table.select (and_ (

(Report.c.diseaseStatus == None),

or_ ((Report.c.country == Burma),

(Report.c.country == Myanmar),

),

)

U i f i l t


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Using func in select

Can use func to make arbitrary tests of fields.

report_table.select (func.substr (report_table.c.country, 1) == A).execute()

S l t i d l


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Select via mapped classes

You can select classes instead of tables.

Make queries on session and filter.

report_table.select (func.substr (

Report.c.country, 1) == A).execute()

query = session.query (Report)

reps = query.filter (Report.c.country == Burma)

Odds d ds


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Odds and ends

orm.clear_mappers(): clears all class mappings, usefulduring development

Allegedly slow Have to explicitly manage sessions

Many ways to do things (like SQLObject)

MySQL tools
http://www.aminus.org/blogs/index.php/fumanchu/2007/08/18/storm_sqlalchemy_and_geniusqlhttp://www.aminus.org/blogs/index.php/fumanchu/2007/08/18/storm_sqlalchemy_and_geniusqlhttp://find/http://goback/


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MySQL tools

Graphical tools for administering MySQL. There are many - theseare only based on my personal experience.

Aqua Data Studio ($?) CocoaMySQL (OSX, maybe the best)

iSQL (cross platform)

Resources


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Resources

SQLAlchemy

SQLAlchemy tutorial SQLAlchemy talk slides
http://www.sqlalchemy.org/http://www.rmunn.com/sqlalchemy-tutorial/tutorial.htmlhttp://cleverdevil.org/elixirtalk/slides.pdfhttp://cleverdevil.org/elixirtalk/slides.pdfhttp://www.rmunn.com/sqlalchemy-tutorial/tutorial.htmlhttp://www.sqlalchemy.org/http://find/http://goback/

Documents

SQL Alchemy Advanced Python Nbn 2007