sqlalchemy 在 postgres 中使用 inheritance

Question

in an attempt to learn sqlalchemy (and python), i am trying to duplicate an already existing project, but am having trouble figuring out sqlalchemy and inheritance with postgres.

here is an example of what our postgres database does (obviously, this is simplified):

CREATE TABLE system (system_id SERIAL PRIMARY KEY, 
                     system_name VARCHAR(24) NOT NULL);
CREATE TABLE file_entry(file_entry_id SERIAL, 
                        file_entry_msg VARCHAR(256) NOT NULL, 
                        file_entry_system_name VARCHAR(24) REFERENCES system(system_name) NOT NULL);
CREATE TABLE ops_file_entry(CONSTRAINT ops_file_entry_id_pkey PRIMARY KEY (file_entry_id), 
     CONSTRAINT ops_system_name_check CHECK ((file_entry_system_name = 'ops'::bpchar))) INHERITS (file_entry);
CREATE TABLE eng_file_entry(CONSTRAINT eng_file_entry_id_pkey PRIMARY KEY (file_entry_id),
     CONSTRAINT eng_system_name_check CHECK ((file_entry_system_name = 'eng'::bpchar)) INHERITS (file_entry);
CREATE INDEX ops_file_entry_index ON ops_file_entry USING btree (file_entry_system_id);
CREATE INDEX eng_file_entry_index ON eng_file_entry USING btree (file_entry_system_id);

And then the inserts would be done with a trigger, so that they were properly inserted into the child databases. Something like:

CREATE FUNCTION file_entry_insert_trigger() RETURNS "trigger"
    AS $$
DECLARE 
BEGIN
     IF NEW.file_entry_system_name = 'eng' THEN
        INSERT INTO eng_file_entry(file_entry_id, file_entry_msg, file_entry_type, file_entry_system_name) VALUES (NEW.file_entry_id, NEW.file_entry_msg, NEW.file_entry_type, NEW.file_entry_system_name);
     ELSEIF NEW.file_entry_system_name = 'ops' THEN
        INSERT INTO ops_file_entry(file_entry_id, file_entry_msg, file_entry_type, file_entry_system_name) VALUES (NEW.file_entry_id, NEW.file_entry_msg, NEW.file_entry_type, NEW.file_entry_system_name);
     END IF;
     RETURN NULL;
 END;
 $$ LANGUAGE plpgsql;

in summary, i have a parent table with a foreign key to another table. then i have 2 child tables that exist, and the inserts are done based upon a given value. in my example above, if file_entry_system_name is 'ops', then the row goes into the ops_file_entry table; 'eng' goes into eng_file_entry_table. we have hundreds of children tables in our production environment, and considering the amount of data, it really speeds things up, so i would like to keep this same structure. i can query the parent, and as long as i give it the right 'system_name', it immediately knows which child table to look into.

my desire is to emulate this with sqlalchemy, but i can't find any examples that go into this much detail. i look at the sql generated by sqlalchemy by examples, and i can tell it is not doing anything similar to this on the database side.

the best i can come up with is something like:

class System(_Base):
    __tablename__ = 'system'
    system_id = Column(Integer, Sequence('system_id_seq'), primary_key = True)
    system_name = Column(String(24), nullable=False)
    def __init(self, name)
        self.system_name = name
class FileEntry(_Base):
    __tablename__ = 'file_entry'
    file_entry_id = Column(Integer, Sequence('file_entry_id_seq'), primary_key=True)
    file_entry_msg = Column(String(256), nullable=False)
    file_entry_system_name = Column(String(24), nullable=False, ForeignKey('system.system_name'))
    __mapper_args__ = {'polymorphic_on': file_entry_system_name}
    def __init__(self, msg, name)
        self.file_entry_msg = msg
        self.file_entry_system_name = name
class ops_file_entry(FileEntry):
    __tablename__ = 'ops_file_entry'
    ops_file_entry_id = Column(None, ForeignKey('file_entry.file_entry_id'), primary_key=True)
    __mapper_args__ = {'polymorphic_identity': 'ops_file_entry'}

in the end, what am i missing? how do i tell sqlalchemy to associate anything that is inserted into FileEntry with a system name of 'ops' to go to the 'ops_file_entry' table? is my understanding way off?

some insight into what i should do would be amazing.

Answer 1

You just create a new instance of ops_file_entry (shouldn't this be OpsFileEntry ?), add it into the session, and upon flush, one row will be inserted into table file_entry as well as table ops_file_entry .

You don't need to set the file_entry_system_name attribute, nor the trigger.

Answer 2

I don't really know python or sqlalchemy, but I figured I'd give it a shot for old times sake. ;)

Have you tried basically setting up your own trigger at the application level? Something like this might work:

from sqlalchemy import event, orm

def my_after_insert_listener(mapper, connection, target):
    # set up your constraints to store the data as you want
    if target.file_entry_system_name = 'eng'
        # do your child table insert
    elseif target.file_entry_system_name = 'ops'
        # do your child table insert
    #…

mapped_file_entry_class = orm.mapper(FileEntry, 'file_entry')
# associate the listener function with FileEntry,
# to execute during the "after_insert" hook
event.listen(mapped_file_entry_class, 'after_insert', my_after_insert_listener)

I'm not positive, but I think target (or perhaps mapper ) should contain the data being inserted.

Events (esp. after_create) and mapper will probably be helpful.