Querying many-to-many in SQL Alchemy

Question

I have two two tables that are related with a third relationship table. Basically, I have many fits, which can use any number of tags, and I'm trying to query the fits tagged with a certain tag using the tagID.

Tag.py:

tags_table = Table("tags", saveddata_meta,
                            Column("tagID", Integer, primary_key = True),
                            Column("name", String, nullable = False, unique=True))

mapper(Tag, tags_table)

Fit.py (stripped down)

fits_table = Table("fits", saveddata_meta,
                         Column("ID", Integer, primary_key = True),
                         Column("ownerID", ForeignKey("users.ID"), nullable = True, index = True),
                         Column("name", String, nullable = False),
                         Column("timestamp", Integer, nullable = False))

fit_tags = Table("fit_tags", saveddata_meta,
                            Column("fitID", Integer, ForeignKey("fits.ID"), primary_key = True),
                            Column("tagID", Integer, ForeignKey("tags.tagID"), primary_key = True))

mapper(Fit, fits_table,
       properties = {
                     "_Fit__tags" : relation(Tag, secondary=fit_tags, backref="fits"),
                     })

And here's the function that I am trying to use:

def getFitsWithTag(tagID, ownerID=None, where=None, eager=None):
    """
    Get all the fits tag with tagID
    If no user is passed, do this for all users.
    """
    if isinstance(tagID, int):
        if ownerID is not None and not isinstance(ownerID, int):
            raise TypeError("OwnerID must be integer")

        filter = Tag.tagID == tagID

        if ownerID is not None:
            filter = and_(filter, Fit.ownerID == ownerID)

        filter = processWhere(filter, where)
        eager = processEager(eager)
        with sd_lock:
            fits = saveddata_session.query(Fit).options(*eager).filter(filter).all()
    else:
        raise TypeError("TagID must be integer")
    return fits

I can access the tags of a fit via Fit._tags. However, I am not sure how to query for the fits based on tag. I've tried many values for filter that I've come across while trying to figure it out.

Answer 1

Add following line to your query:

def getFitsWithTag(tagID, ownerID=None, where=None, eager=None):
    # ...
    fits = (saveddata_session.query(Fit)
            .join(fit_tags).filter(fit_tags.c.tagID == tagID) # @note: add me
            .options(*eager).filter(filter)
            ).all()
    # ...
    return fits

Also, why such a strange name (_Fit__tags) for a relationship? I guess it is coming from django. Since sqlalchemy does not give special meanings in those, you can name is just tags.