PostGraphile v5 Plugins

Create custom plugins to extend PostGraphile's functionality using hooks.

Official Documentation

•Plugin Introduction: https://postgraphile.org/postgraphile/next/plugins
•Extending Schema: https://postgraphile.org/postgraphile/next/extending

When to Apply

Use this skill when:

•Adding custom query or mutation fields
•Detecting naming conflicts in multi-schema setups
•Adding metadata or introspection queries
•Modifying the build process
•Extending GraphQL types

Plugin Structure

typescript

import type { GraphileConfig } from 'graphile-config';

export const MyPlugin: GraphileConfig.Plugin = {
  name: 'MyPlugin',
  version: '1.0.0',
  description: 'What this plugin does',

  // Inflection overrides
  inflection: {
    replace: {
      // Override inflectors
    },
  },

  // Schema hooks
  schema: {
    hooks: {
      // Build-time hooks
    },
    entityBehavior: {
      // Behavior modifications
    },
  },
};

When Do Plugins Run?

Important: Plugins run at BUILD TIME, not on every request. The schema is built once and cached. This means:

•Hooks only execute during schema generation
•Logging in hooks only appears at startup
•Changes require server restart

Schema Hooks

Available Hooks

Hook	When it runs	Use case
`init`	Start of build	Collect metadata, setup
`build`	After init	Analyze codecs, detect conflicts
`GraphQLObjectType_fields`	When adding fields to types	Add custom fields
`GraphQLSchema`	Final schema	Modify complete schema

Hook: init

Runs at the start of schema building. Good for collecting metadata:

typescript

schema: {
  hooks: {
    init(_, build) {
      const { pgRegistry } = build.input;
      
      // Access all tables/resources
      for (const resource of Object.values(pgRegistry.pgResources)) {
        console.log('Table:', resource.name);
      }
      
      return _;
    },
  },
},

Hook: build

Runs after init. Good for analyzing the schema:

typescript

schema: {
  hooks: {
    build(build) {
      // Access inflection
      const inflection = build.inflection;
      
      // Access codecs (types)
      for (const codec of Object.values(build.input.pgRegistry.pgCodecs)) {
        if (codec.attributes) {
          const typeName = inflection.tableType(codec);
          console.log('Type:', typeName);
        }
      }
      
      return build;
    },
  },
},

Hook: GraphQLObjectType_fields

Add or modify fields on GraphQL types:

typescript

schema: {
  hooks: {
    GraphQLObjectType_fields(fields, build, context) {
      const { Self } = context;
      
      // Only modify Query type
      if (Self.name !== 'Query') {
        return fields;
      }
      
      // Add a custom field
      return {
        ...fields,
        hello: {
          type: build.graphql.GraphQLString,
          resolve() {
            return 'world';
          },
        },
      };
    },
  },
},

Common Plugin Patterns

Conflict Detector Plugin

Detect naming conflicts between tables in different schemas:

typescript

import type { GraphileConfig } from 'graphile-config';

export const ConflictDetectorPlugin: GraphileConfig.Plugin = {
  name: 'ConflictDetectorPlugin',
  version: '1.0.0',

  schema: {
    hooks: {
      build(build) {
        const codecsByName = new Map<string, Array<{ schema: string; table: string }>>();

        for (const codec of Object.values(build.input.pgRegistry.pgCodecs)) {
          if (!codec.attributes || codec.isAnonymous) continue;

          const pgExtensions = codec.extensions?.pg as { schemaName?: string } | undefined;
          const schemaName = pgExtensions?.schemaName || 'unknown';
          const graphqlName = build.inflection.tableType(codec);

          if (!codecsByName.has(graphqlName)) {
            codecsByName.set(graphqlName, []);
          }
          codecsByName.get(graphqlName)!.push({
            schema: schemaName,
            table: codec.name,
          });
        }

        // Log conflicts
        for (const [graphqlName, tables] of codecsByName) {
          if (tables.length > 1) {
            const locations = tables.map(t => `${t.schema}.${t.table}`).join(', ');
            console.warn(`NAMING CONFLICT: "${graphqlName}" from: ${locations}`);
          }
        }

        return build;
      },
    },
  },
};

Custom Mutation Plugin

Add a custom mutation:

typescript

import type { GraphileConfig } from 'graphile-config';
import {
  GraphQLObjectType,
  GraphQLNonNull,
  GraphQLString,
  GraphQLInputObjectType,
} from 'graphql';

export const CustomMutationPlugin: GraphileConfig.Plugin = {
  name: 'CustomMutationPlugin',
  version: '1.0.0',

  schema: {
    hooks: {
      GraphQLObjectType_fields(fields, build, context) {
        const { Self } = context;
        if (Self.name !== 'Mutation') return fields;

        const SendEmailInput = new GraphQLInputObjectType({
          name: 'SendEmailInput',
          fields: {
            to: { type: new GraphQLNonNull(GraphQLString) },
            subject: { type: new GraphQLNonNull(GraphQLString) },
            body: { type: new GraphQLNonNull(GraphQLString) },
          },
        });

        const SendEmailPayload = new GraphQLObjectType({
          name: 'SendEmailPayload',
          fields: {
            success: { type: new GraphQLNonNull(build.graphql.GraphQLBoolean) },
            messageId: { type: GraphQLString },
          },
        });

        return {
          ...fields,
          sendEmail: {
            type: SendEmailPayload,
            args: {
              input: { type: new GraphQLNonNull(SendEmailInput) },
            },
            async resolve(_parent, args, context) {
              const { to, subject, body } = args.input;
              // Implement email sending logic
              return { success: true, messageId: 'msg-123' };
            },
          },
        };
      },
    },
  },
};

Combining Inflection and Hooks

typescript

export const CompletePlugin: GraphileConfig.Plugin = {
  name: 'CompletePlugin',
  version: '1.0.0',

  // Inflection overrides
  inflection: {
    replace: {
      _schemaPrefix(_previous, _options, _details) {
        return '';
      },
    },
  },

  // Schema hooks
  schema: {
    hooks: {
      build(build) {
        console.log('Schema building...');
        return build;
      },
    },
    
    // Behavior modifications
    entityBehavior: {
      pgResourceUnique: {
        override: {
          provides: ['myBehavior'],
          callback(behavior, [_resource, unique]) {
            if (!unique.isPrimary) {
              return [behavior, '-single'];
            }
            return behavior;
          },
        },
      },
    },
  },
};

Creating a Preset from Plugins

typescript

export const MyPluginPreset: GraphileConfig.Preset = {
  plugins: [
    ConflictDetectorPlugin,
    MetaSchemaPlugin,
    CustomMutationPlugin,
  ],
};

// Usage
const preset: GraphileConfig.Preset = {
  extends: [
    PostGraphileAmberPreset,
    MyPluginPreset,
  ],
};

Troubleshooting

Issue	Solution
Hook not called	Check plugin is in preset's `plugins` array
Changes not visible	Restart server (schema is cached)
Type errors	Import types from `graphile-config`
Can't access build properties	Check hook signature and available properties
Conflict with other plugins	Check plugin order in `plugins` array

Source Code References

•graphile-build hooks: https://github.com/graphile/crystal/tree/main/graphile-build/graphile-build
•PgTablesPlugin (example): https://github.com/graphile/crystal/blob/main/graphile-build/graphile-build-pg/src/plugins/PgTablesPlugin.ts

References

•PostGraphile v5 Plugin Docs: https://postgraphile.org/postgraphile/next/plugins
•PostGraphile v5 Extending Schema: https://postgraphile.org/postgraphile/next/extending
•See graphile-v5-inflection skill for inflector customization
•See graphile-v5-behaviors skill for behavior modifications
•See graphile-v5-debugging skill for debugging plugins