SQLAlchemy Invalid Foreign Key with PostgreSQL but not SQLite

Question

I can create the following tables in an SQLite database:

class Article(db.Model):
    id = db.Column(db.Integer, primary_key=True)

class Paragraph(db.Model):
    article_id = db.Column(db.ForeignKey('article.id'), primary_key=True)
    index = db.Column(db.Integer, primary_key=True)

class Summary(db.Model):
    article_id = db.Column(db.ForeignKey('article.id'), primary_key=True)
    paragraph_index = db.Column(db.ForeignKey('paragraph.index'), primary_key=True)
    level = db.Column(db.Integer, primary_key=True)

However, when I create the same tables in PostgreSQL, I get the following error:

sqlalchemy.exc.ProgrammingError: (psycopg2.errors.InvalidForeignKey) there is no unique constraint matching given keys for referenced table "paragraph"

Removing Summary.paragraph_index allows me to create the tables, so it appears that the issue lies here. However, the column does reference a primary key, which I feel should satisfy the requirement.

Adding unique=True to Paragraph.index also allows me to create the tables. I can then remove the unique constraint using pgAdmin and have everything work fine. However, this seems improper.

I have also tried:

• declaring the relationship via the ORM
• declaring ForeignKeyConstraint in Summary table args
• declaring UniqueConstraint in Paragraph table args for the primary key columns