Setting up a GraphQL schema using an existing database¶
Table of contents
Introduction¶
When you have an existing database with a schema already present, you don’t need to create tables or views or run DDL queries through the Hasura console.
All you need to do is indicate to Hasura GraphQL engine which tables and views you want to expose over GraphQL and how they are connected to each other so that you can query them as a “graph”.
Step 1: Track tables/views¶
Tracking a table or a view means telling Hasura GraphQL engine that you want to expose that table/view over GraphQL.
To track a table or a view¶
- Head to the
Data -> Schema
section of the console. - Under the heading
Untracked Tables/Views
, click on theTrack
button next to the table/view name.
To track the table and expose it over the GraphQL API, add it to the tables.yaml
file in the metadata
directory as follows:
- table:
schema: public
name: <table name>
Apply the metadata by running:
hasura metadata apply
To track a table and expose it over the GraphQL API, use the track_table metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "track_table",
"args": {
"schema": "public",
"name": "<table name>"
}
}
To track all tables and views present in the database¶
- Head to the
Data -> Schema
section of the console. - Under the heading
Untracked Tables/Views
, click theTrack All
button.
To track all tables and expose them over the GraphQL API, add them to the tables.yaml
file in the metadata
directory as follows:
- table:
schema: public
name: <table-name-1>
- table:
schema: public
name: <table-name-2>
To automate this, you could add the tables in a loop through a script.
Apply the metadata by running:
hasura metadata apply
To track all tables and expose them over the GraphQL API, use the track_table metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "bulk",
"args": [
{
"type": "track_table",
"args": {
"schema": "public",
"name": "<table-name-1>"
}
},
{
"type": "track_table",
"args": {
"schema": "public",
"name": "<table-name-2>"
}
}
]
}
To automate this, you could add the track_table
requests to the bulk
request in a loop through a script.
Step 2: Track foreign-keys¶
Tracking a foreign-key means creating a relationship between the tables involved in the foreign-key.
To track a foreign-key between two tables in the database¶
- Head to the
Data -> Schema
section of the console. - Click on a table involved in the foreign-key and head to the
Relationships
tab. - You should see a suggested relationship based on the foreign-key. Click
Add
, give a name to your relationship (this will be the name of the nested object in the GraphQL query), and hitSave
to create the relationship. - Repeat with the other table involved in the foreign-key.
To track a relationship and expose it over the GraphQL API, add it to the tables.yaml
file in the metadata
directory as follows:
Object relationship
- table:
schema: public
name: <table name>
object_relationships:
- name: <relationship name>
using:
foreign_key_constraint_on: <reference column>
Array relationship
- table:
schema: public
name: <table name>
array_relationships:
- name: <relationship name>
using:
foreign_key_constraint_on:
column: <reference column>
table:
schema: public
name: <reference table name>
Apply the metadata by running:
hasura metadata apply
Object relationship
You can create an object relationship by using the create_object_relationship metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "create_object_relationship",
"args": {
"table": "<table name>",
"name": "<relationship name>",
"using": {
"foreign_key_constraint_on": "<reference column>"
}
}
}
Array relationship
You can create an array relationship by using the create_array_relationship metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "create_array_relationship",
"args": {
"table": "<table name>",
"name": "<relationship name>",
"using": {
"foreign_key_constraint_on" : {
"table" : "<reference table name>",
"column" : "<reference column>"
}
}
}
}
To track all the foreign-keys of all tables in the database¶
- Head to the
Data -> Schema
section of the console. - Under the heading
Untracked foreign-key relations
, click theTrack All
button to automatically create relationships based on the foreign-keys.
To track all relationships and expose them over the GraphQL API, add them to the tables.yaml
file in the metadata
directory as follows:
Object relationship
- table:
schema: public
name: <table name>
object_relationships:
- name: <relationship name>
using:
foreign_key_constraint_on: <reference column>
Array relationship
- table:
schema: public
name: <table name>
array_relationships:
- name: <relationship name>
using:
foreign_key_constraint_on:
column: <reference column>
table:
schema: public
name: <reference table name>
To automate this, you could add the relationships in a loop through a script.
Apply the metadata by running:
hasura metadata apply
You can create multiple relationships by using the create_object_relationship metadata API and the create_array_relationship metadata API:
POST /v1/query HTTP/1.1
Content-Type: application/json
X-Hasura-Role: admin
{
"type": "bulk",
"args": [
{
"type": "create_object_relationship",
"args": {
"table": "<table name>",
"name": "<relationship name>",
"using": {
"foreign_key_constraint_on": "<reference column>"
}
}
},
{
"type": "create_array_relationship",
"args": {
"table": "<table name>",
"name": "<relationship name>",
"using": {
"foreign_key_constraint_on" : {
"table" : "<reference table name>",
"column" : "<reference column>"
}
}
}
}
]
}
To automate this, you could add the create relationships requests to the bulk
request in a loop through a script.
Relationship nomenclature
In this case, Hasura GraphQL engine will automatically generate relationship names (the names of the nested objects in the GraphQL query) based on the table names and the foreign-key names.
The name is generated in the following format:
- For object relationships:
singular of foreignTableName
- For array relationships:
plural of foreignTableName
For example, for the foreign-key article.author_id -> author.id
, the relationship names will be
author
for article
table and articles
for author
table.
In case a field with the generated name already exists, a new name will be generated of the form:
camel case of (singular/plural of foreignTableName + _by_ + foreignKeyColumnName)
Note that, this is just an arbitrary naming convention chosen by Hasura to ensure the generation of unique relationship names. You can choose to rename your relationships to anything you wish. You can change the relationship names with a name of your choice as shown in Renaming relationships.