Modelling many-to-many table relationships

Introduction

A many-to-many relationship between two tables can be established by creating a table typically called as bridge/junction/join table and adding foreign-key constraints from it to the original tables.

Say we have the following two tables in our database schema:

articles (
  id SERIAL PRIMARY KEY,
  title TEXT
  ...
)

tags (
  id SERIAL PRIMARY KEY,
  tag_value TEXT
  ...
)

These two tables are related via a many-to-many relationship. i.e:

• an article can have many tags
• a tag has many articles

Step 1: Set up a table relationship in the database

This many-to-many relationship can be established in the database by:

1. Creating a bridge table called article_tag with the following structure:

   article_tag (
     id SERIAL PRIMARY KEY
     article_id INT
     tag_id INT
     UNIQUE (article_id, tag_id)
     ...
   )

Note

If you can have multiple rows linking the same article and tag in your model, you can skip the unique constraint on (article_id, tag_id)

2. Adding foreign key constraints in the article_tag table for:

   • the articles table using the article_id and id columns of the tables respectively
   • the tags table using the tag_id and id columns of the tables respectively

The table article_tag sits between the two tables involved in the many-to-many relationship and captures possible permutations of their association via the foreign keys.

Step 2: Set up GraphQL relationships

To access the nested objects via the GraphQL API, create the following relationships:

• Array relationship, article_tags from articles table using article_tag :: article_id -> id
• Object relationship, tag from article_tag table using tag_id -> tags :: id
• Array relationship, tag_articles from tags table using article_tag :: tag_id -> id
• Object relationship, article from article_tag table using article_id -> articles :: id

Query using many-to-many relationships

We can now:

• fetch a list of articles with their tags:

GraphiQL

Query Variables

Request Headers

• fetch a list of tags with their articles:

GraphiQL

Query Variables

Request Headers

Insert using many-to-many relationships

We can now:

• insert an article with tags where the tag might already exist (assume unique value for tag):

GraphiQL

Query Variables

Request Headers

• insert a tag with articles where the tag might already exist (assume unique value for tag):

GraphiQL

Query Variables

Request Headers

Note

You can avoid the on_conflict clause if you will never have conflicts.

Fetching relationship information

The intermediate fields article_tags & tag_articles can be used to fetch extra information about the relationship. For example, you can have a column like tagged_at in the article_tag table which you can fetch as follows:

GraphiQL

Query Variables

Request Headers

Flattening a many-to-many relationship query

In case you would like to flatten the above queries and avoid the intermediate fields article_tags & tag_articles, you can create the following views additionally and then query using relationships created on these views:

CREATE VIEW article_tags_view AS
  SELECT article_id, tags.*
    FROM article_tag LEFT JOIN tags
      ON article_tag.tag_id = tags.id

CREATE VIEW tag_articles_view AS
  SELECT tag_id, articles.*
    FROM article_tag LEFT JOIN articles
      ON article_tag.article_id = articles.id

Now create the following relationships:

• Array relationship, tags from the articles table using article_tags_view :: article_id -> id
• Array relationship, articles from the tags table using tag_articles_view :: tag_id -> id

We can now:

• fetch articles with their tags without an intermediate field:

GraphiQL

Query Variables

Request Headers

• fetch tags with their articles without an intermediate field:

GraphiQL

Query Variables

Request Headers

Note

We do not recommend this flattening pattern of modelling as this introduces an additional overhead of managing permissions and relationships on the newly created views. e.g. You cannot query for the author of the nested articles without setting up a new relationship to the authors table from the tag_articles_view view.

In our opinion, the cons of this approach seem to outweigh the pros.