sqlalchemy Oracle schema - oracle

Question

I should extract data from an oracle database.
How can I find out which schema are defined in the database?
When I do not define any schema in the description of Metadata(), I find no tables.
thanks for your help,

Answer 1

Default Oracle schema matches the username that was used in Oracle connection.
If you don't see any tables - it means the tables are created in another schema.
Looks like you have two questions here:
1) about Oracle schemas - how to find schema and tables in Oracle
2) about SQLAlchemy reflections - how to specify Oracle schema for table
You can find answer for the first question in many places. I.e. here: https://stackoverflow.com/a/2247758/1296661
Answering second question:
Table class constructor has schema argument to specify table's schema if it is different from default user's schema. See more here
http://docs.sqlalchemy.org/en/rel_0_7/core/schema.html#sqlalchemy.schema.Table
Here is the python code to answer second question. You will need to setup db connection and table name values to match your case:
from sqlalchemy import Table
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine
engine = create_engine('oracle://<user_name>:<password>#<hostname>:1521/<instance name>', echo=True)
Base = declarative_base()
reflected_table = Table('<Table name>',
Base.metadata,
autoload=True,
autoload_with=engine,
schema='<Schema name other then user_name>')
print [c.name for c in reflected_table.columns]
p = engine.execute("SELECT OWNER,count(*) table_count FROM ALL_OBJECTS WHERE OBJECT_TYPE = 'TABLE' GROUP BY OWNER");
for r in p:
print r
Good luck with using sqlschema and reflection feature - it is a lot of fun. You get your python program working with existing database almost without defining schema information in your program.
I'm using this feature with oracle db in production - the only thing I have to define were relations between tables explicitly setting foreign and primary keys.

Answer 2

I found two other ways to set an alternate Oracle schema.
Using MetaData and passing it into the creation of the SqlAlchemy instance or engine.
Or using __table_args__ when defining your models and pass in {'schema': 'other_schema' }
http://docs.sqlalchemy.org/en/latest/core/metadata.html#sqlalchemy.schema.MetaData.params.schema
How to specify PostgreSQL schema in SQLAlchemy column/foreign key mixin?
But both of these don't help the foreign key calls. For those, you still have to manually prefix with other_schema. before the table name.
The snippet below is in the context of a Flask app with Flask-SqlAlchemy in use.
from sqlalchemy import schema
from flask_sqlalchemy import SQLAlchemy
import os
os.environ['TNS_ADMIN'] = os.path.join( 'oracle_tools', 'network', 'admin')
oracle_connection_string = 'oracle+cx_oracle://{username}:{password}#{tnsname}'.format(
username='user',
password='pass',
tnsname='TNS_SPACE',
)
oracle_schema = 'user_schema_1'
foreign_key_prefix = oracle_schema + '.'
app.config['SQLALCHEMY_DATABASE_URI'] = oracle_connection_string
oracle_db_metadata = schema.MetaData(schema=oracle_schema)
db = SQLAlchemy(app, metadata=oracle_db_metadata)
user_groups_table = db.Table(db_table_prefix + 'user_groups', db.Model.metadata,
db.Column('...', db.Integer, db.ForeignKey(foreign_key_prefix + 'group.id')),
# schema=oracle_schema # Not necessary because of the use of MetaData above
)
class User(db.Model):
__tablename__ = 'user'
# __table_args__ = {'schema': oracle_schema } # Not necessary because of the use of MetaData above
user_name = db.Column('USER_NAME', db.String(250))
And... two other related links:
https://github.com/mitsuhiko/flask-sqlalchemy/issues/172
How to specify PostgreSQL schema in SQLAlchemy column/foreign key mixin?
Hope that helps.