Library mode is the most popular way of running PostGraphile; it gives more power than using the CLI (see CLI usage), but is much easier to setup and more fully featured than Schema-only Usage.

Details of the configuration options can be found below.

PostGraphile supports usage in library mode within various Node server frameworks. There are two ways of using the PostGraphile library mode inside a server framework: very concisely as middleware, or via the more verbose individual route handlers introduced in PostGraphile v4.10.0. The below table summarises the support of these modes by each of the frameworks we support, and the following documentation sections detail how to use them.

HTTP Middleware

To mount a PostGraphile instance on your own web server there is an export postgraphile from the postgraphile package that can be used as HTTP middleware for Node's HTTP server, Express, Connect, Koa and Fastify v2 (but not Fastify v3). Usage is generally of the form app.use(postgraphile(...)).

Additional frameworks such as Restify and Fastify are also supported since PostGraphile v4.10.0 via the route handlers method detailed below.

To use PostGraphile with express, for instance, a small example would be:

const express = require("express");
const { postgraphile } = require("postgraphile");

const app = express();

app.use(
  postgraphile(
    process.env.DATABASE_URL || "postgres://user:pass@host:5432/dbname",
    "public",
    {
      watchPg: true,
      graphiql: true,
      enhanceGraphiql: true,
    }
  )
);

app.listen(process.env.PORT || 3000);

Or to use it with the built-in http module:

const http = require("http");
const { postgraphile } = require("postgraphile");

http
  .createServer(
    postgraphile(
      process.env.DATABASE_URL || "postgres://user:pass@host:5432/dbname",
      "public",
      {
        watchPg: true,
        graphiql: true,
        enhanceGraphiql: true,
      }
    )
  )
  .listen(process.env.PORT || 3000);

Route Handlers

Introduced in PostGraphile v4.10.0.

If your server does not support the preferred middleware approach of mounting the PostGraphile library, then you can mount PostGraphile's route handlers individually. This looks slightly different depending on which server you're using (see the table above for links to examples for each server), but broadly it follows the following pattern (this example uses Fastify v3).

First we create the PostGraphile middleware:

import { postgraphile } from "postgraphile";

const middleware = postgraphile(process.env.DATABASE_URL, "public", {
  /* ... options here ... */
});

And your server:

import Fastify from "fastify";

const app = Fastify({});

Next we need an adaptor to convert a generic PostGraphile route handler into a handler that's suitable for your given server framework. We provide the following out of the box:

• PostGraphileResponseNode - for Node, Express, Connect, Nest, Restify, and Fastify v2 (NOT v3)
• PostGraphileResponseKoa - for Koa
• PostGraphileResponseFastify3 - for Fastify v3

You can also make your own by sub-classing PostGraphileResponse, for help with this ping @Benjie on Discord.

Note that some frameworks expect you to indicate failure by rejecting a promise, others expect you to pass an error object to the next(err) callback. PostGraphile's handler rejects on error, so if your framework (e.g. Express) requires next(err) you should add .catch(next) to the handler call.

import { PostGraphileResponseFastify3 } from "postgraphile";

// Converts a PostGraphile handler to a Fastify v3 handler
const convertHandler = handler => (request, reply) =>
  handler(new PostGraphileResponseFastify3(request, reply));

/* Other frameworks:

// Converts a PostGraphile handler to an Express handler
const convertHandler = handler => (req, res, next) =>
  handler(new PostGraphileResponseNode(req, res, next)).catch(next);

// Converts a PostGraphile handler to a Koa handler
const convertHandler = handler => (ctx, next) =>
  handler(new PostGraphileResponseKoa(ctx, next));
*/

Then we use the properties present on the PostGraphile middleware to mount our route handlers. The properties available include:

• options - the PostGraphile options your server is using

• route strings:

  • graphqlRoute (e.g. /graphql) - the URL path the GraphQL endpoint should be mounted at
  • graphiqlRoute (e.g. /graphiql) - the URL path the GraphiQL (GraphQL IDE) endpoint should be mounted at
  • eventStreamRoute (e.g. /graphql/stream) - the URL path the EventStream for PostGraphile watch mode should be mounted at (relates to the X-GraphQL-Event-Stream header)

• request handlers

  • graphqlRouteHandler - handles GraphQL POST and OPTIONS requests
  • graphiqlRouteHandler - handles GET requests to retrieve the GraphiQL interface
  • faviconRouteHandler - serves the PostGraphile favicon
  • eventStreamRouteHandler - serves the PostGraphile watch-mode event stream

For Fastify, this might look something like:

// OPTIONS requests, for CORS/etc
app.options(
  middleware.graphqlRoute,
  convertHandler(middleware.graphqlRouteHandler)
);

// This is the main middleware
app.post(
  middleware.graphqlRoute,
  convertHandler(middleware.graphqlRouteHandler)
);

// GraphiQL, if you need it
if (middleware.options.graphiql) {
  if (middleware.graphiqlRouteHandler) {
    app.head(
      middleware.graphiqlRoute,
      convertHandler(middleware.graphiqlRouteHandler)
    );
    app.get(
      middleware.graphiqlRoute,
      convertHandler(middleware.graphiqlRouteHandler)
    );
  }
  // Remove this if you don't want the PostGraphile logo as your favicon!
  if (middleware.faviconRouteHandler) {
    app.get("/favicon.ico", convertHandler(middleware.faviconRouteHandler));
  }
}

// If you need watch mode, this is the route served by the
// X-GraphQL-Event-Stream header; see:
// https://github.com/graphql/graphql-over-http/issues/48
if (middleware.options.watchPg) {
  if (middleware.eventStreamRouteHandler) {
    app.options(
      middleware.eventStreamRoute,
      convertHandler(middleware.eventStreamRouteHandler)
    );
    app.get(
      middleware.eventStreamRoute,
      convertHandler(middleware.eventStreamRouteHandler)
    );
  }
}

For Nest, this might look something like:

import { Controller, Get, Post, Req, Next, Res } from '@nestjs/common';
import { Request, Response } from 'express';
import { PostGraphileResponseNode } from 'postgraphile';
import { middleware } from './postgraphile.middleware';

@Controller('/')
export class PostGraphileController {
  @Get(middleware.graphiqlRoute)
  graphiql (@Req() request: Request, @Res() response: Response, @Next() next) {
    middleware.graphiqlRouteHandler(new PostGraphileResponseNode(request, response, next));
  }

  @Post(middleware.graphqlRoute)
  graphql (@Req() request: Request, @Res() response: Response, @Next() next) {
    middleware.graphqlRouteHandler(new PostGraphileResponseNode(request, response, next));
  }
}

IMPORTANT: although it's tempting to add your handlers with explicitly written paths, e.g. app.post('/graphql', ...), it's better to use the relevant middleware properties such as middleware.graphqlRoute to ensure that PostGraphile is expecting the GraphQL endpoint to be in the same place that you're expecting.

IMPORTANT: it's advised that you also look through the full-flavoured "rum-and-raisin" example server for your framework; depending on what other middleware/plugins you're using (e.g. compression) you may need to add some tiny workarounds for common issues (most notably relating to the Event Stream from watch mode).

Recommended options

As PostGraphile evolves, we add more features; however we can't always enable these features by default as they may be breaking changes. There are also options that may have security repurcussions. For this reason, many features are behind flags. We understand this page is very long, so we've included some default option sets you might like to use:

We recommend you install the @graphile-contrib/pg-simplify-inflector plugin.

For Development

const postgraphileOptions = {
  subscriptions: true,
  watchPg: true,
  dynamicJson: true,
  setofFunctionsContainNulls: false,
  ignoreRBAC: false,
  showErrorStack: "json",
  extendedErrors: ["hint", "detail", "errcode"],
  appendPlugins: [require("@graphile-contrib/pg-simplify-inflector")],
  exportGqlSchemaPath: "schema.graphql",
  graphiql: true,
  enhanceGraphiql: true,
  allowExplain(req) {
    // TODO: customise condition!
    return true;
  },
  enableQueryBatching: true,
  legacyRelations: "omit",
  pgSettings(req) {
    /* TODO */
  },
};

For Production

const postgraphileOptions = {
  subscriptions: true,
  retryOnInitFail: true,
  dynamicJson: true,
  setofFunctionsContainNulls: false,
  ignoreRBAC: false,
  extendedErrors: ["errcode"],
  appendPlugins: [require("@graphile-contrib/pg-simplify-inflector")],
  graphiql: false,
  enableQueryBatching: true,
  disableQueryLog: true, // our default logging has performance issues, but do make sure you have a logging system in place!
  legacyRelations: "omit",
  pgSettings(req) {
    /* TODO */
  },
};

API: postgraphile(pgConfig, schemaName, options)

The postgraphile middleware factory function takes three arguments, all of which are optional. The below options are valid for postgraphile@4.12.3.

• pgConfig: Specifies the PostgreSQL database you wish to connect to. You may pass a PostgreSQL connection string, a configuration object to pass to the pg.Pool constructor, or a pg.Pool instance. Note: pg.Pool has a default pool size of 10, to increase this you use the configuration object/pg.Pool instance approach and pass a different value as max.
• schemaName: A string, or array of strings, which specifies the PostgreSQL schema(s) you to expose via PostGraphile; defaults to 'public'

• options: An object containing other miscellaneous options. Options include:

  • watchPg: When true, PostGraphile will update the GraphQL API whenever your database schema changes. This feature requires some changes to your database in the form of the postgraphile_watch schema; PostGraphile will try to add this itself but requires DB superuser privileges to do so. If PostGraphile can't install it, you can do so manually. PostGraphile will not drop the schema when it exits, to remove it you can execute: DROP SCHEMA postgraphile_watch CASCADE;
  • retryOnInitFail: When false (default), PostGraphile will exit if it fails to build the initial schema (for example if it cannot connect to the database, or if there are fatal naming conflicts in the schema). When true, PostGraphile will keep trying to rebuild the schema indefinitely, using an exponential backoff between attempts, starting at 100ms and increasing up to 30s delay between retries. When a function, the function will be called passing the error and the number of attempts, and it should return true to retry, false to permanently abort trying.
  • ownerConnectionString: Connection string to use to connect to the database as a privileged user (e.g. for setting up watch fixtures, logical decoding, etc).
  • subscriptions: Enable GraphQL websocket transport support for subscriptions (you still need a subscriptions plugin currently)
  • live: [EXPERIMENTAL] Enables live-query support via GraphQL subscriptions (sends updated payload any time nested collections/records change)
  • websockets: Choose which websocket transport libraries to use. Use commas to define multiple. Defaults to ['v0', 'v1'] if subscriptions or live are true, [] otherwise
  • websocketOperations: Toggle which GraphQL websocket transport operations are supported: 'subscriptions' or 'all'. Defaults to subscriptions
  • websocketMiddlewares: [EXPERIMENTAL] If you're using websockets (subscriptions || live) then you may want to authenticate your users using sessions or similar. You can pass some simple middlewares here that will be executed against the websocket connection in order to perform authentication. We current only support Express (not Koa, Fastify, Restify, etc) middlewares here.
  • pgDefaultRole: The default Postgres role to use. If no role was provided in a provided JWT token, this role will be used.
  • dynamicJson: By default, JSON and JSONB fields are presented as strings (JSON encoded) from the GraphQL schema. Setting this to true (recommended) enables raw JSON input and output, saving the need to parse / stringify JSON manually.
  • setofFunctionsContainNulls: If none of your RETURNS SETOF compound_type functions mix NULLs with the results then you may set this false to reduce the nullables in the GraphQL schema.
  • classicIds: Enables classic ids for Relay support. Instead of using the field name nodeId for globally unique ids, PostGraphile will instead use the field name id for its globally unique ids. This means that table id columns will also get renamed to rowId.
  • disableDefaultMutations: Setting this to true will prevent the creation of the default mutation types & fields. Database mutation will only be possible through Postgres functions.
  • ignoreRBAC: Set false (recommended) to exclude fields, queries and mutations that are not available to any possible user (determined from the user in connection string and any role they can become); set this option true to skip these checks and create GraphQL fields and types for everything. The default is true, in v5 the default will change to false.
  • ignoreIndexes: Set false to exclude filters, orderBy, and relations that would be expensive to access due to missing indexes. Changing this from true to false is a breaking change, but false to true is not. The default is true.
  • includeExtensionResources: By default, tables and functions that come from extensions are excluded from the generated GraphQL schema as general applications don't need them to be exposed to the end user. You can use this flag to include them in the generated schema (not recommended).
  • showErrorStack: Enables adding a stack field to the error response. Can be either the boolean true (which results in a single stack string) or the string json (which causes the stack to become an array with elements for each line of the stack). Recommended in development, not recommended in production.
  • extendedErrors: Extends the error response with additional details from the Postgres error. Can be any combination of ['hint', 'detail', 'errcode']. Default is [].
  • handleErrors: Enables ability to modify errors before sending them down to the client. Optionally can send down custom responses. If you use this then showErrorStack and extendedError may have no effect.
  • appendPlugins: An array of Graphile Engine schema plugins to load after the default plugins.
  • prependPlugins: An array of Graphile Engine schema plugins to load before the default plugins (you probably don't want this).
  • replaceAllPlugins: The full array of Graphile Engine schema plugins to use for schema generation (you almost definitely don't want this!).
  • skipPlugins: An array of Graphile Engine schema plugins to skip.
  • readCache: A file path string or an object. Reads cached values to improve startup time (you may want to do this in production).
  • writeCache: A file path string. Writes computed values to local cache file so startup can be faster (do this during the build phase).
  • exportJsonSchemaPath: Enables saving the detected schema, in JSON format, to the given location. The directories must exist already, if the file exists it will be overwritten.
  • exportGqlSchemaPath: Enables saving the detected schema, in GraphQL schema format, to the given location. The directories must exist already, if the file exists it will be overwritten.
  • sortExport: If true, lexicographically (alphabetically) sort exported schema for more stable diffing.
  • graphqlRoute: The endpoint the GraphQL executer will listen on. Defaults to /graphql.
  • eventStreamRoute: The endpoint the watch-mode EventStream will be mounted on (only appropriate when watchPg is specified). Defaults to ${graphqlRoute}/stream.
  • externalGraphqlRoute: The URL to the GraphQL endpoint for embedding into the GraphiQL client. We attempt to infer this (for many servers it is the same as graphqlRoute), but you may need to specify it manually if you mount PostGraphile behind a URL-rewriting proxy, or mount PostGraphile on a subpath in certain Node.js servers.
  • externalEventStreamRoute: As with externalGraphqlRoute, but for eventStreamRoute rather than graphqlRoute. This is also used for the X-GraphQL-Event-Stream header.
  • graphiqlRoute: The endpoint the GraphiQL query interface will listen on (NOTE: GraphiQL will not be enabled unless the graphiql option is set to true). Defaults to /graphiql.
  • externalUrlBase: DEPRECATED - use externalGraphqlRoute and externalEventStreamRoute instead. If you are using watch mode, or have enabled GraphiQL, and you either mount PostGraphile under a path, or use PostGraphile behind some kind of proxy that puts PostGraphile under a subpath (or both!) then you must specify this setting so that PostGraphile can figure out it's external URL. (e.g. if you do app.use('/path/to', postgraphile(...))), which is not officially supported, then you should pass externalUrlBase: '/path/to'.) This setting should never end in a slash (/). To specify that the external URL is the expected one, either omit this setting or set it to the empty string ''.
  • graphiql: Set this to true to enable the GraphiQL interface.
  • graphiqlCredentials: Set this to change the way GraphiQL handles credentials. By default this is set to same-origin.
  • enhanceGraphiql: Set this to true to add some enhancements to GraphiQL; intended for development usage only (automatically enables with subscriptions and live).
  • enableCors: Enables some generous CORS settings for the GraphQL endpoint. There are some costs associated when enabling this, if at all possible try to put your API behind a reverse proxy.
  • bodySizeLimit: Set the maximum size of HTTP request bodies that can be parsed (default 100kB). The size can be given as a human-readable string, such as '200kB' or '5MB' (case insensitive).
  • enableQueryBatching: [Experimental] Enable the middleware to process multiple GraphQL queries in one request.
  • jwtSecret: The secret for your JSON web tokens. This will be used to verify tokens in the Authorization header, and signing JWT tokens you return in procedures.
  • jwtPublicKey: The public key to verify the JWT when signed with RS265 or ES256 algorithms.
  • jwtVerifyOptions: Options with which to perform JWT verification - see https://github.com/auth0/node-jsonwebtoken#jwtverifytoken-secretorpublickey-options-callback If 'audience' property is unspecified, it will default to ['postgraphile']; to prevent audience verification set it explicitly to null.
  • jwtSignOptions: Options with which to perform JWT signing - see https://github.com/auth0/node-jsonwebtoken#jwtsignpayload-secretorprivatekey-options-callback
  • jwtRole: An array of (strings) path components that make up the path in the jwt from which to extract the postgres role. By default, the role is extracted from token.role, so the default value is ['role']. e.g. { iat: 123456789, creds: { local: { role: "my_role" } } } the path would be token.creds.local.role i.e. ['creds', 'local', 'role']
  • jwtPgTypeIdentifier: The Postgres type identifier for the compound type which will be signed as a JWT token if ever found as the return type of a procedure. Can be of the form: my_schema.my_type. You may use quotes as needed: "my-special-schema".my_type.
  • jwtAudiences: [DEPRECATED] The audience to use when verifing the JWT token. Deprecated, use jwtVerifyOptions.audience instead.
  • legacyRelations: Some one-to-one relations were previously detected as one-to-many - should we export 'only' the old relation shapes, both new and old but mark the old ones as 'deprecated' (default), or 'omit' (recommended) the old relation shapes entirely.
  • legacyJsonUuid: ONLY use this option if you require the v3 typenames 'Json' and 'Uuid' over 'JSON' and 'UUID'.
  • disableQueryLog: Turns off GraphQL query logging. By default PostGraphile will log every GraphQL query it processes along with some other information. Set this to true (recommended in production) to disable that feature.
  • pgSettings: A plain object specifying custom config values to set in the PostgreSQL transaction (accessed via current_setting('my.custom.setting')) or an (optionally asynchronous) function which will return the same (or a Promise to the same) based on the incoming web request (e.g. to extract session data).
  • allowExplain: [Experimental] Determines if the 'Explain' feature in GraphiQL can be used to show the user the SQL statements that were executed. Set to a boolean to enable all users to use this, or to a function that filters each request to determine if the request may use Explain. DO NOT USE IN PRODUCTION unless you're comfortable with the security repurcussions of doing so.
  • additionalGraphQLContextFromRequest: Some Graphile Engine schema plugins may need additional information available on the context argument to the resolver - you can use this function to provide such information based on the incoming request - you can even use this to change the response [experimental], e.g. setting cookies.
  • pluginHook: [experimental] Plugin hook function, enables functionality within PostGraphile to be expanded with plugins. Generate with makePluginHook(plugins) passing a list of plugin objects.
  • simpleCollections: Should we use relay pagination, or simple collections? "omit" (default) - relay connections only, "only" (not recommended) - simple collections only (no Relay connections), "both" - both.
  • queryCacheMaxSize: Max query cache size in bytes (extremely approximate, not accurate at all). Default 50000000 (~50MB). Set to 0 to disable.

The following options are not part of PostGraphile core, but are available from the @graphile/pg-pubsub subscriptions plugin (formerly the paid "supporter" plugin, but now fully free and open source - please consider sponsoring us!):

• simpleSubscriptions: set this to true to add simple subscription support
• subscriptionAuthorizationFunction [fn] set this to the name (excluding arguments/parentheses) of a PG function to call to check user is allowed to subscribe

The following options are not part of PostGraphile core, but are available from the Pro plugin - see Go Pro! for more information.

• options:

  • readOnlyConnection pro spon ⚡️[experimental] set this to a PostgreSQL connection string to use for read-only queries (i.e. not mutations)
  • defaultPaginationCap pro spon ⚡️[experimental] integer, ensure all connections have first/last specified and are no large than this value (default: 50), set to -1 to disable; override via smart comment @paginationCap 50
  • graphqlDepthLimit pro spon ⚡️[experimental] integer, validate GraphQL queries are no deeper than the specified int (default: 16), set to -1 to disable
  • graphqlCostLimit pro spon ⚡️[experimental] integer, only allows queries with a computed cost below the specified int (default: 1000), set to -1 to disable
  • exposeGraphQLCost pro spon boolean, if true (default) then the calculated query cost will be exposed on the resulting payload

Exposing HTTP request data to PostgreSQL

pgSettings function

Using the pgSettings functionality mentioned above you can extend the data made available through current_setting(...) within PostgreSQL. Instead of passing an object you can pass an (optionally asynchronous) function which will be executed for each request, and the results merged in with the other settings PostGraphile automatically adds to the request.

For example:

export postgraphile(process.env.DATABASE_URL, schemaName, {
  pgSettings: async req => ({
    'user.id': `${req.session.passport.user}`,
    'http.headers.x-something': `${req.headers['x-something']}`,
    'http.method': `${req.method}`,
    'http.url': `${req.url}`,
    //...
  }),
})

create function get_x_something() returns text as $$
  select current_setting('http.headers.x-something', true)::text;
$$ language sql stable;

Everything returned by pgSettings is applied to the current session with set_config($key, $value, true); note that set_config only supports string values so it is best to only feed pgSettings string values (we'll convert other values using the String constructor function, which may not have the effect you intend.

You can use pgSettings to define variables that your Postgres functions/policies depend on, or to tweak internal Postgres settings. When adding variables for your own usage, the keys must contain either one or two period (.) characters, and the prefix (the bit before the first period) must not be used by any Postgres extension. Variables without periods will be interpreted as internal Postgres settings, such as role, and will be applied by Postgres. All settings are automatically reset when the transaction completes. Here's an example of switching the user into the Postgres 'visitor' role, and applying the application setting jwt.claims.user_id:

export postgraphile(process.env.DATABASE_URL, schemaName, {
  pgSettings: async req => ({
    'role': 'visitor',
    'jwt.claims.user_id': `${req.user.id}`,
    //...
  }),
})

CREATE FUNCTION get_current_user() RETURNS TEXT AS $$
  SELECT current_user;
$$ LANGUAGE SQL STABLE;

{
  getCurrentUser # returns visitor
}

Making HTTP data available to resolvers

additionalGraphQLContextFromRequest function

additionalGraphQLContextFromRequest is an optionally asynchronous function passed the req and res request and response objects from your HTTP library. The result returned from the function is merged into the GraphQL context object which is passed as the third argument to every GraphQL resolver.

If you're using Koa then you can get back to the Koa context using const ctx = req._koaCtx.

Be careful to not clash with internal context keys such as pgClient and jwtClaims. For the absolute best future compatibility, we recommend that you prefix your context keys with your initials or similar.

Example:

const postgraphileOptions = {
  // ... other options here ...

  async additionalGraphQLContextFromRequest(req, res) {
    // You can perform asynchronous actions here if you need to; for example
    // looking up the current user in the database.

    // Return here things that your resolvers need
    return {
      // Return the current user from Passport.js or similar
      user: req.user,

      // Add a helper to get a header
      getHeader(name) {
        return req.get(name);
      },

      // Give access to the database-owner PostgreSQL pool, for example to
      // perform privileged actions
      rootPgPool,
    };
  },
};

It's not a good idea to return direct access to the req or res objects from additionalGraphQLContextFromRequest(req, res) as it binds the context too tightly to the request lifecycle, and that's not compatible with local usage of the schema, or with usage over other transports such as websockets for realtime. Instead, add helpers to get/set the data you need.

Mounting PostGraphile middleware under a subpath

This isn't officially supported; however it should work in most cases. If you're mounting under a subpath in express then app.use("/path/to", postgraphile()) should work automatically without requiring any options. If you're using an external proxy then you must supply the base URL so that PostGraphile knows where to tell the browser the assets are located. This is all so that PostGraphile can reference different parts of itself correctly, such as the location for the watch stream to put in the header, or the GraphQL endpoint for GraphiQL to connect to.

A really complex use case of this would be this example:

// Assuming you combine both Express subpath AND an external
// proxy which mounts your express app at `/myproxypath`, you
// should provide options like this:
app.use(
  "/path/to",
  postgraphile(db, schemas, {
    externalUrlBase: "/myproxypath/path/to",
    graphqlRoute: "/graphql",
    graphiql: true,
    graphiqlRoute: "/graphiql",
  })
);
// Then you can load GraphiQL at `/myproxypath/path/to/graphiql`
// and it will know to connect to GraphQL at
// `/myproxypath/path/to/graphql`

Composing PostGraphile middleware with other middleware

Some use cases might require to mount other middleware before PostGraphile, for instance if requests to the GraphQL endpoint should be subject to an additional authorization / authentication mechanism.

With express, such a composition can be done like the following:

// Example middleware, does nothing
function authMiddleware(req, res, next) {
  //...
  next();
}
// Mount middleware on the GraphQL endpoint
app.use("/graphql", authMiddleware);
// Mount PostGraphile after this middleware
app.use(postgraphile(connectionString, schema, options));

This example uses Express, but a similar approach is possible with other Node.js webservers.

handleErrors

Optionally you may specify a handleErrors function as part of the PostGraphile options. This function will be passed the list of errors that occurred during the GraphQL operation (if any occurred) and must return this same list, or a list that will be used to replace the errors.

In GraphQL.js, all errors are wrapped in a GraphQLError object. You can get to the original error that was thrown from a resolver via the originalError property on the GraphQLError, however some errors might be GraphQL query validation or other error types that don't have an originalError. Further some errors might be provided by PostGraphile itself rather than GraphQL, so might not be GraphQLError objects.

The original error could be thrown from any number of places (especially if you're using plugins) but if it is thrown from PostgreSQL, then it might be a pg DatabaseError error.

Errors are detailed in the GraphQL Spec, so be sure that the errors you return from handleErrors conform to these rules.

You can find an example of handleErrors in Graphile Starter.