A many-to-many table with SQLAlchemy which links a table (Paper) to the same table (Paper)

Question

I'm using SQLModel, which has SQLAlchemy underneath. I am trying to create a many-to-many relationship using a join table to link the model Paper back to other Paper objects via PaperSimilarLink. It's a "similar papers" function.

class PaperSimilarLink(SQLModel, table=True):
    paper_id: Optional[int] = Field(
        default=None, foreign_key="paper.id", primary_key=True
    )
    similar_id: Optional[int] = Field(
        default=None, foreign_key="paper.id", primary_key=True
    )

class Paper(SQLModel, HasRelatedObjects, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    similar: List["Paper"] = Relationship(
        back_populates="similar",
        link_model=PaperSimilarLink,
        sa_relationship=dict(
            primary_join=PaperSimilarLink.paper_id,
            secondary_join=PaperSimilarLink.similar_id,
        ),
    )

without adding the primary_join and secondary_join to the Paper.similar column, I get the below error:

sqlalchemy.exc.AmbiguousForeignKeysError: Could not determine join condition between parent/child tables on relationship PaperSection.similar - there are multiple foreign key paths linking the tables via secondary table 'papersectionsimilarlink'. Specify the 'foreign_keys' argument, providing a list of those columns which should be counted as containing a foreign key reference from the secondary table to each of the parent and child tables.

After adding it, the code does run but I then get an error trying to access Paper.similar:

AttributeError: 'Paper' object has no attribute 'similar'

Might be useful to see the postgres table inspection on postgres

----------------+-------------------+-----------+----------+-----------------------------------
 figures        | json              |           |          | 
 id             | integer           |           | not null | nextval('paper_id_seq'::regclass)
 title          | character varying |           | not null | 
 link           | character varying |           | not null | 
 pub_date       | character varying |           | not null | 
 doi            | character varying |           | not null | 
 abstract       | character varying |           | not null | 
 pdf_link       | character varying |           |          | 
 opensummary_id | integer           |           |          | 
Indexes:
    "paper_pkey" PRIMARY KEY, btree (id)
    "paper_doi_key" UNIQUE CONSTRAINT, btree (doi)
Foreign-key constraints:
    "paper_opensummary_id_fkey" FOREIGN KEY (opensummary_id) REFERENCES opensummary(id)

...skipping 1 line
    TABLE "authorpaperlink" CONSTRAINT "authorpaperlink_paper_id_fkey" FOREIGN KEY (paper_id) REFE
RENCES paper(id)
    TABLE "medrivixpaper" CONSTRAINT "medrivixpaper_paper_id_fkey" FOREIGN KEY (paper_id) REFERENC
ES paper(id)
    TABLE "paperreferencelink" CONSTRAINT "paperreferencelink_paper_id_fkey" FOREIGN KEY (paper_id
) REFERENCES paper(id)
    TABLE "papersection" CONSTRAINT "papersection_paper_id_fkey" FOREIGN KEY (paper_id) REFERENCES
 paper(id)
    TABLE "papersimilarlink" CONSTRAINT "papersimilarlink_paper_id_fkey" FOREIGN KEY (paper_id) RE
FERENCES paper(id)
    TABLE "papersimilarlink" CONSTRAINT "papersimilarlink_similar_id_fkey" FOREIGN KEY (similar_id
) REFERENCES paper(id)