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SQLAlchemy - query with secondary table with reflected Base

I work with a pre-existing database and SQLAlchemy. I reflect all tables like
Base = automap_base()
Base.prepare(engine, reflect=True)
Then all the classes can be accesed with e.g. a_table = Base.classes.A. From documentation I understand, that if a table has only two columns, both foreign keys, then the table
is assumed to be a “secondary” table, and will not be mapped directly
So the secondary class is not among the classes in the Base and
The two (or one, for self-referential) external tables to which the
Table refers to are matched to the classes to which they will be
mapped, if any. If mapped classes for both sides are located, a
many-to-many bi-directional relationship() / backref() pair is created
between the two classes.
But I cannot figure out how to use that in a query. To clarify my scenario, I have tables like shown below and I want to get (depending on A_id) C_ids for B, where B_id is main_B_id in A.
But whatever query I try it never sees the other table. I tried some workarounds, but I guess there must be some simple way how to achieve that and I just missed some part of the documentation. Any hint would be highly appreciated, thanks!

SQLAlchemy MetaData.reflect() vs. automap_base.prepare()

It seems to me that MetaData.reflect() and sqlalchemy.ext.automap.prepare() tables should be able to be used interchangeably for many use cases, but they can't be.
The metadata.tables['mytable'] into conn.execute(select(...)) returns a sqlalchemy.engine.cursor.CursorResult and your iterator gets the columns directly (eg x.columnA).
But automap_base().classes.mytable into the same conn.execute(select(...)) returns a sqlalchemy.engine.result.ChunkedIteratorResult and you need x.mytable.columnA to get at the column.
The sqlalchemy.engine.Result() documention says as much:
New in version 1.4: The Result object provides a completely updated
usage model and calling facade for SQLAlchemy Core and SQLAlchemy ORM.
In Core, it forms the basis of the CursorResult object which replaces
the previous ResultProxy interface. When using the ORM, a higher level
object called ChunkedIteratorResult is normally used.
Can I generically convert one to the other? That is, some wrapper that works for every table without needing the table name?
What's the best futureproof way to do this? I want my code to be forward-looking to sqlalchemy 2.0. Does that mean I should move away from either automap or MetaData?
sqlalchemy 1.4.35

This is the difference between the Core and the ORM.
select() from a Table vs. ORM class
While the SQL generated in these examples looks the same whether we
invoke select(user_table) or select(User), in the more general case
they do not necessarily render the same thing, as an ORM-mapped class
may be mapped to other kinds of “selectables” besides tables. The
select() that’s against an ORM entity also indicates that ORM-mapped
instances should be returned in a result, which is not the case when
SELECTing from a Table object.
Don't hesitate to use the ORM. It's higher level, pythonic, cool, and automap is ORM.

in database related document, parent and child is refer to what?

that makes sense that we refer to the table that uses values from another table as a child(this column is defined as a foreign key), in this regard the other table is parent. this makes more sense in one to many relations.
one record in the table(parent) has a relationship with one or more records in another table(child). in short terms one parent has many child.
in this answer from StackOverflow these were mentioned.
but in this section of SQLAlchemy document that talks about many to one relationship, they are placing the ForeingKey in the parent table . what does it mean?
also in this section of SQLALchey document that talks about many to many relationship between two table, and implementing this behavior with Asociation object rather than Association table, they referring to one of the tables as parent and another one as child what does it mean?

Populating related table in SqlAlchemy ORM

So I have two table in a one-to-many relationship. When I make a new row of Table1, I want to populate Table2 with the related rows. However, this population actually involves computing the Table2 rows, using data in other related tables.
What's a good way to do that using the ORM layer? That is, assuming that that the Table1 mappings are created through the ORM, where/how should I call the code to populate Table2?
I thought about using the after_insert hook, but i want to have a session to pass to the population method.
Thanks.

You can use the before_flush or after_flush hook, it provides a session. You then check session.new objects for newly created models (tip: use isinstance(object, ModelClass)) and do your work here.
In fact, SQLAlchemy recommends before_flush for general on flush changes.
Mapper-level flush events only allow very limited operations, on attributes local to the row being operated upon only, as well as allowing any SQL to be emitted on the given Connection. Please read fully the notes at Mapper-level Events for guidelines on using these methods; generally, the SessionEvents.before_flush() method should be preferred for general on-flush changes.

After asking around in #sqlalchemy IRC, it was pointed out that this could be done using ORM-level relationships in an before_flush event listener.
It was explained that when you add a mapping through a relationship, the foreign key is automatically filled on flush, and the appropriate insert statement generated by the ORM.

Discovering referers to SQLAlchemy object

I have a lot of model classes with ralations between them with a CRUD interface to edit. The problem is that some objects can't be deleted since there are other objects refering to them. Sometimes I can setup ON DELETE rule to handle this case, but in most cases I don't want automatic deletion of related objects till they are unbound manually. Anyway, I'd like to present editor a list of objects refering to currently viewed one and highlight those that prevent its deletion due to FOREIGN KEY constraint. Is there a ready solution to automatically discover referers?
Update
The task seems to be quite common (e.g. django ORM shows all dependencies), so I wonder that there is no solution to it yet.
There are two directions suggested:
Enumerate all relations of current object and go through their backref. But there is no guarantee that all relations have backref defined. Moreover, there are some cases when backref is meaningless. Although I can define it everywhere I don't like doing this way and it's not reliable.
(Suggested by van and stephan) Check all tables of MetaData object and collect dependencies from their foreign_keys property (the code of sqlalchemy_schemadisplay can be used as example, thanks to stephan's comments). This will allow to catch all dependencies between tables, but what I need is dependencies between model classes. Some foreign keys are defined in intermediate tables and have no models corresponding to them (used as secondary in relations). Sure, I can go farther and find related model (have to find a way to do it yet), but it looks too complicated.
Solution
Below is a method of base model class (designed for declarative extention) that I use as solution. It is not perfect and doesn't meet all my requirements, but it works for current state of my project. The result is collected as dictionary of dictionaries, so I can show them groupped by objects and their properties. I havn't decided yet whether it's good idea, since the list of referers sometimes is huge and I'm forced to limit it to some reasonable number.
def _get_referers(self):
db = object_session(self)
cls, ident = identity_key(instance=self)
medatada = cls.__table__.metadata
result = {}
# _mapped_models is my extension. It is collected by metaclass, so I didn't
# look for other ways to find all model classes.
for other_class in medatada._mapped_models:
queries = {}
for prop in class_mapper(other_class).iterate_properties:
if not (isinstance(prop, PropertyLoader) and \
issubclass(cls, prop.mapper.class_)):
continue
query = db.query(prop.parent)
comp = prop.comparator
if prop.uselist:
query = query.filter(comp.contains(self))
else:
query = query.filter(comp==self)
count = query.count()
if count:
queries[prop] = (count, query)
if queries:
result[other_class] = queries
return result
Thanks to all who helped me, especially stephan and van.

SQL: I have to absolutely disagree with S.Lott' answer.
I am not aware of out-of-the-box solution, but it is definitely possible to discover all the tables that have ForeignKey constraints to a given table. One needs to use properly the INFORMATION_SCHEMA views such as REFERENTIAL_CONSTRAINTS, KEY_COLUMN_USAGE, TABLE_CONSTRAINTS, etc. See SQL Server example. With some limitations and extensions, most versions of new relational databases support INFORMATION_SCHEMA standard. When you have all the FK information and the object (row) in the table, it is a matter of running few SELECT statements to get all other rows in other tables that refer to given row and prevent it from being deleted.
SqlAlchemy: As noted by stephan in his comment, if you use orm with backref for relations, then it should be quite easy for you to get the list of parent objects that keep reference to the object you are trying to delete, because those objects are basically mapped properties of your object (child1.Parent).
If you work with Table objects of sql alchemy (or not always use backref for relations), then you would have to get values of foreign_keys for all the tables, and then for all those ForeignKeys call references(...) method, providing your table as a parameter. In this way you will find all the FKs (and tables) that have reference to the table your object maps to. Then you can query all the objects that keep reference to your object by constructing the query for each of those FKs.

In general, there's no way to "discover" all of the references in a relational database.
In some databases, they may use declarative referential integrity in the form of explicit Foreign Key or Check constraints.
But there's no requirement to do this. It can be incomplete or inconsistent.
Any query can include a FK relationship that is not declared. Without the universe of all queries, you can't know the relationships which are used but not declared.
To find "referers" in general, you must actually know the database design and have all queries.

For each model class, you can easily see if all its one-to-many relations are empty simply by asking for the list in each case and seeing how many entries it contains. (There is probably a more efficient way implemented in terms of COUNT, too.) If there are any foreign keys relating to the object, and you have your object relations set up correctly, then at least one of these lists will be non-zero in length.