graphql-engine/docs/graphql/core/guides/data-modelling/many-to-many.rst

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.. meta::
:description: Model many-to-many relationships in Hasura
:keywords: hasura, docs, schema, relationship, many-to-many, n-m
2019-12-26 15:05:37 +03:00
.. _many_to_many_modelling:
Modelling many-to-many table relationships
==========================================
.. contents:: Table of contents
:backlinks: none
:depth: 1
:local:
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:
.. code-block:: sql
article (
id SERIAL PRIMARY KEY,
title TEXT
...
)
tag (
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:
.. code-block:: sql
article_tag (
article_id INT
tag_id INT
PRIMARY KEY (article_id, tag_id)
...
)
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2. Adding **foreign key constraints** from the ``article_tag`` table to:
- the ``article`` table using the ``article_id`` and ``id`` columns of the tables respectively
- the ``tag`` 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
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permutations of their association via the foreign keys.
Step 2: Set up GraphQL relationships
------------------------------------
To access the nested objects via the GraphQL API, :ref:`create the following relationships <create_relationships>`:
- Array relationship, ``article_tags`` from ``article`` table using ``article_tag :: article_id -> id``
- Object relationship, ``tag`` from ``article_tag`` table using ``tag_id -> tag :: id``
- Array relationship, ``tag_articles`` from ``tag`` table using ``article_tag :: tag_id -> id``
- Object relationship, ``article`` from ``article_tag`` table using ``article_id -> article :: id``
Query using many-to-many relationships
--------------------------------------
We can now:
- fetch a list of ``articles`` with their ``tags``:
.. graphiql::
:view_only:
:query:
query {
article {
id
title
article_tags {
tag {
id
tag_value
}
}
}
}
:response:
{
"data": {
"article": [
{
"id": 1,
"title": "sit amet",
"article_tags": [
{
"tag": {
"id": 1,
"tag_value": "mystery"
}
},
{
"tag": {
"id": 2,
"tag_value": "biography"
}
}
]
},
{
"id": 2,
"title": "a nibh",
"article_tags": [
{
"tag": {
"id": 2,
"tag_value": "biography"
}
},
{
"tag": {
"id": 5,
"tag_value": "technology"
}
}
]
}
]
}
}
- fetch a list of ``tags`` with their ``articles``:
.. graphiql::
:view_only:
:query:
query {
tag {
id
tag_value
tag_articles {
article {
id
title
}
}
}
}
:response:
{
"data": {
"tag": [
{
"id": 1,
"tag_value": "mystery",
"tag_articles": [
{
"article": {
"id": 1,
"title": "sit amet"
}
}
]
},
{
"id": 2,
"tag_value": "biography",
"tag_articles": [
{
"article": {
"id": 1,
"title": "sit amet"
}
},
{
"article": {
"id": 2,
"title": "a nibh"
}
}
]
}
]
}
}
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::
:view_only:
:query:
mutation insertArticleWithTags {
insert_article(objects: [
{
title: "Article 1",
content: "Article 1 content",
author_id: 1,
article_tags: {
data: [
{
tag: {
data: {
value: "Recipes"
},
on_conflict: {
constraint: tag_value_key,
update_columns: [value]
}
}
}
{
tag: {
data: {
value: "Cooking"
},
on_conflict: {
constraint: tag_value_key,
update_columns: [value]
}
}
}
]
}
}
]) {
returning {
title
article_tags {
tag {
value
}
}
}
}
}
:response:
{
"data": {
"insert_article": {
"returning": [
{
"title": "Article 1",
"article_tags": [
{
"tag": {
"value": "Recipes"
}
},
{
"tag": {
"value": "Cooking"
}
}
]
}
]
}
}
}
- insert a ``tag`` with ``articles`` where the ``tag`` might already exist (assume unique ``value`` for ``tag``):
.. graphiql::
:view_only:
:query:
mutation insertTagWithArticles {
insert_tag(objects: [
{
value: "Recipes",
article_tags: {
data: [
{
article: {
data: {
title: "Article 1",
content: "Article 1 content",
author_id: 1
}
}
},
{
article: {
data: {
title: "Article 2",
content: "Article 2 content",
author_id: 1
}
}
}
]
}
}
],
on_conflict: {
constraint: tag_value_key,
update_columns: [value]
}
) {
returning {
value
article_tags {
article {
title
}
}
}
}
}
:response:
{
"data": {
"insert_tag": {
"returning": [
{
"value": "Recipes",
"article_tags": [
{
"article": {
"title": "Article 1"
}
},
{
"article": {
"title": "Article 2"
}
}
]
}
]
}
}
}
.. 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::
:view_only:
:query:
query {
article {
id
title
article_tags {
tagged_at
tag {
id
tag_value
}
}
}
}
:response:
{
"data": {
"article": [
{
"id": 1,
"title": "sit amet",
"article_tags": [
{
"tagged_at": "2018-11-19T18:01:17.292828+05:30",
"tag": {
"id": 1,
"tag_value": "mystery"
}
},
{
"tagged_at": "2018-11-18T18:01:17.292828+05:30",
"tag": {
"id": 3,
"tag_value": "romance"
}
}
]
},
{
"id": 2,
"title": "a nibh",
"article_tags": [
{
"tagged_at": "2018-11-19T15:01:17.292828+05:30",
"tag": {
"id": 5,
"tag_value": "biography"
}
},
{
"tagged_at": "2018-11-16T14:01:17.292828+05:30",
"tag": {
"id": 3,
"tag_value": "romance"
}
}
]
}
]
}
}
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 :ref:`create the following views <custom_views>` additionally and then
query using relationships created on these views:
.. code-block:: sql
CREATE VIEW article_tags_view AS
SELECT article_id, tag.*
FROM article_tag LEFT JOIN tag
ON article_tag.tag_id = tag.id
CREATE VIEW tag_articles_view AS
SELECT tag_id, article.*
FROM article_tag LEFT JOIN article
ON article_tag.article_id = article.id
Now :ref:`create the following relationships <create_relationships>`:
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- Array relationship, ``tags`` from the ``article`` table using ``article_tags_view :: article_id -> id``
- Array relationship, ``articles`` from the ``tag`` table using ``tag_articles_view :: tag_id -> id``
We can now:
- fetch articles with their tags without an intermediate field:
.. graphiql::
:view_only:
:query:
query {
article {
id
title
tags {
id
tag_value
}
}
}
:response:
{
"data": {
"article": [
{
"id": 1,
"title": "sit amet",
"tags": [
{
"id": 1,
"tag_value": "mystery"
},
{
"id": 3,
"tag_value": "romance"
}
]
},
{
"id": 2,
"title": "a nibh",
"tags": [
{
"id": 5,
"tag_value": "biography"
},
{
"id": 3,
"tag_value": "romance"
}
]
}
]
}
}
- fetch tags with their articles without an intermediate field:
.. graphiql::
:view_only:
:query:
query {
tag {
id
tag_value
articles {
id
title
}
}
}
:response:
{
"data": {
"tag": [
{
"id": 1,
"tag_value": "mystery",
"articles": [
{
"id": 1,
"title": "sit amet"
}
]
},
{
"id": 2,
"tag_value": "biography",
"articles": [
{
"id": 1,
"title": "sit amet"
},
{
"id": 2,
"title": "a nibh"
}
]
}
]
}
}
.. 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 ``author`` table from the ``tag_articles_view`` view.
In our opinion, the cons of this approach seem to outweigh the pros.