How to Connect to a PostgreSQL Database

How to use a Postgres database as a data source for your GraphQL API

StepZen supports MySQL, Postgres, MSSQL, and Snowflake databases. In this tutorial, we'll see how to connect to a Postgres database:

See also the GraphQL directives Reference.

Getting started

To generate your GraphQL on a PostgreSQL backend via stepzen import postgresql, see Getting started with a PostgreSQL Database documentation.

Extending Your GraphQL API

Any Query or Mutation field in your StepZen GraphQL schema can be annotated with the @dbquery directive to connect to a database backend.

@dbquery (type: String!, query: String, dml: enum, table: String, configuration: String!)

@dbquery enables you to connect a MySQL, PostgreSQL, MSSQL, or Snowflake database. For more, see the GraphQL directives Reference.

Directive Arguments

The available arguments to the @dbquery directive are:

type

This argument is required, and specifies the type of database to query. Supported values are mysql, postgresql, mssql, and snowflake.

table

The value of this argument is the name of the database table to be queried. While this value is optional, one of either table or query must be specified.

Using the table argument is the equivalent of writing select * from [table]. StepZen assumes that the column names of the underlying database table will match the field names of the GraphQL type of the annotated field. Thus, if the table has a name column, it will populate the name field of the GraphQL type.

If the annotated field has arguments, they are used to construct the WHERE clause of the SQL query. For example, let's look at the following annotated field:

customerById (id: ID!): Customer
  @dbquery (
    type: "postgresql"
    table: "customers"
    configuration: "postgresql_config"
  )

This passes the following database query to the database specified by the postgresql_config configuration (See below for more about configurations).

SELECT "id", "name", "email", "creditCard" FROM "customers" WHERE "id" = $1

where, id, name, email and creditCard are the columns of the Postgres table customers that match the fields of the Customer type. If the annotated field has multiple arguments, they are combined in the SQL WHERE clause with an AND.

Note: Postgres' comparisons are case-sensitive, so care must be given to the naming of arguments to match the Postgres columns.

query

The value of this argument is the SQL query whose results are used to populate the sub-fields of the annotated field. While this value is optional, one of either table or query must be specified. The query argument is useful when you need to perform a complex query, or when the table column names and GraphQL type fields do not match. For example:

customerById (id: ID!): Customer
  @dbquery (
    type: "postgresql",
    query: 'SELECT "id", "full_name" AS "name", "email" FROM "customer" WHERE "id" = $1 AND "creditCard" is not NULL',
    configuration: "postgresql_config"
  )

The above directive executes the specified SQL query on the database specified by the postgresql_config. The SQL query both renames full_name to name so it matches the field name in the GraphQL type Customer, and retrieves only those customers who have a credit card.

Note: Unquoted column names in Postgres are converted to lower case, so care must be given to match the referenced columns in the specified query to the Postgres columns.

configuration

This argument is required and identifies which configuration in the config.yaml file should be used to connect to the database. A PostgreSQL database configuration contains the uri for connecting to your database, and will look similar to this:

configurationset:
  - configuration:
      name: postgresql_config
      uri: postgresql://username:password@address:port/dbname

In this example, postgresql_config is the named configuration that will be referenced by the configuration property of @dbquery as configuration: postgresql_config.

To learn more about the configuration settings for connecting to your PostgreSQL database, see Postgres Configuration.

dml

This argument is optional and is used when the annotated field is a mutation. Its value is an enum that specifies the type of mutation being performed. Valid values are INSERT and DELETE. Cannot be set when query is set.

Note: Ensure you declare the value without surrounding it in quotes, since dml is an enum and not a string.

The following is an example of a mutation with a annotated field whose dml argument value is INSERT:

type Mutation {
  addCustomerById(id: ID!, name: String!, email: String!): Customer
    @dbquery(
      type: "postgresql"
      table: "customer"
      dml: INSERT
      configuration: "postgresql_config"
    )
}

The selection of the addCustomer field of this mutation results in the execution of an insert statement followed by a select statement in the database backend, adding the customer and using the inserted values to populate the returned GraphQL Customer type:

INSERT INTO "customer"("id", "name", "email") VALUES ($1, $2, $3)
SELECT "id", "name", "email" FROM "customer" WHERE "id" = $1 AND "name" = $2 and "email" = $3

Next, let's look at an example of using DELETE:

type Mutation {
  removeCustomerById(id: ID!): Customer
    @dbquery(
      type: "postgresql"
      table: "customer"
      dml: DELETE
      configuration: "postgresql_config"
    )
}

The selection of the removeCustomerById field of this mutation results in the execution of a select statement followed by a delete statement in the PostgreSQL backend, resulting in the removal of the customer with the specified id and using the deleted values to populate the returned GraphQL Customer type:

SELECT "id", "name", "email" FROM "customer" WHERE "id" = $1
DELETE FROM "customer" WHERE "id" = $1