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Schema Inference

This used to be called GraphQL Node Types Creation and should contain info about schema inference and inference > metadata

This documentation isn’t up to date with the latest schema customization changes.

Outdated areas are:

  • the inferObjectStructureFromNodes function doesn’t exist anymore
  • setFieldsOnGraphQLNodeType has been deprecated due to the new createTypes action
  • file node creation has been moved away from setFileNodeRootType

You can help by making a PR to update this documentation.

Gatsby creates a GraphQLObjectType for each distinct node.internal.type that is created during the source-nodes phase. Find out below how this is done.

GraphQL Types for each type of node

When running a GraphQL query, there are a variety of fields that you will want to query. Let’s take an example, say we have the below query:

When GraphQL runs, it will query all file nodes by their relativePath and return the first node that satisfies that query. Then, it will filter down the fields to return by the inner expression. I.e { childMarkdownRemark ... }. The building of the query arguments is covered by the Inferring Input Filters doc. This section instead explains how the inner filter schema is generated (it must be generated before input filters are inferred).

During the sourceNodes phase, let’s say that gatsby-source-filesystem ran and created a bunch of File nodes. Then, different transformers react via onCreateNode, resulting in children of different node.internal.types being created.

There are 3 categories of node fields that we can query.

Fields on the created node object


fields created by setFieldsOnGraphQLNodeType

Each of these categories of fields is created in a different way, explained below.

gqlType Creation

The Gatsby term for the GraphQLObjectType for a unique node type, is gqlType. GraphQLObjectTypes are objects that define the type name and fields. The field definitions are created by the createNodeFields function in build-node-types.js.

An important thing to note is that all gqlTypes are created before their fields are inferred. This allows fields to be of types that haven’t yet been created due to their order of compilation. This is accomplished by the use of fields being a function (basically lazy functions).

The first step in inferring GraphQL Fields is to generate an exampleValue. It is the result of merging all fields of all nodes of the type in question. This exampleValue will therefore contain all potential field names and values, which allows us to infer each field’s types. The logic to create it is in getExampleValues.

With the exampleValue in hand, we can use each of its key/values to infer the Type’s fields (broken down by the 3 categories above).

Fields on the created node object

Fields on the node that were created directly by the source and transform plugins. E.g. for File type, these would be relativePath, size, accessTime etc.

The creation of these fields is handled by the inferObjectStructureFromNodes function in infer-graphql-type.js. Given an object, a field could be in one of 3 sub-categories:

  1. It involves a mapping in gatsby-config.js
  2. Its value is a foreign key reference to some other node (ends in ___NODE)
  3. It’s a plain object or value (e.g. String, number, etc)

Mapping field

Mappings are explained in the gatsby-config.js docs. If the object field we’re generating a GraphQL type for is configured in the gatsby-config mapping, then we handle it specially.

Imagine our top level Type we’re currently generating fields for is MarkdownRemark.frontmatter. And the field we are creating a GraphQL field for is called author. And, that we have a mapping setup of:

The field generation in this case is handled by inferFromMapping. The first step is to find the type that is mapped to. In this case, AuthorYaml. This is known as the linkedType. That type will have a field to link by. In this case name. If one is not supplied, it defaults to id. This field is known as linkedField

Now we can create a GraphQL Field declaration whose type is AuthorYaml (which we look up in list of other gqlTypes). The field resolver will get the value for the node (in this case, the author string), and then search through the React nodes until it finds one whose type is AuthorYaml and whose name field matches the author string.

Foreign Key reference (___NODE)

If not a mapping field, it might instead end in ___NODE, signifying that its value is an ID that is a foreign key reference to another node in Redux. Check out the Source Plugin Tutorial for how this works from a user point of view. Behind the scenes, the field inference is handled by inferFromFieldName.

This is actually quite similar to the mapping case above. We remove the ___NODE part of the field name. E.g. author___NODE would become author. Then, we find our linkedNode. I.e given the example value for author (which would be an ID), we find its actual node in Redux. Then, we find its type in processed types by its internal.type. Note, that also like in mapping fields, we can define the linkedField too. This can be specified via nodeFieldname___NODE___linkedFieldName. E.g. for author___NODE___name, the linkedField would be name instead of id.

Now we can return a new GraphQL Field object, whose type is the one found above. Its resolver searches through all Redux nodes until it finds one with the matching ID. As usual, it also creates a page dependency, from the query context’s path to the node ID.

If the foreign key value is an array of IDs, then instead of returning a Field declaration for a single field, we return a GraphQLUnionType, which is a union of all the distinct linked types in the array.

Plain object or value field

If the field was not handled as a mapping or foreign key reference, then it must be a normal every day field. E.g. a scalar, string, or plain object. These cases are handled by inferGraphQLType.

The core of this step creates a GraphQL Field object, where the type is inferred directly via the result of typeof. E.g. typeof(value) === 'boolean' would result in type GraphQLBoolean. Since these are simple values, resolvers are not defined (graphql-js takes care of that for us).

If however, the value is an object or array, we recurse, using inferObjectStructureFromNodes to create the GraphQL fields.

In addition, Gatsby creates custom GraphQL types for File (types/type-file.js) and Date (types/type-date.js). If the value of our field is a string that looks like a filename or a date (handled by should-infer functions), then we return the appropriate custom type.

Child/Parent fields

Child fields creation

Let’s continue with the File type example. There are many transformer plugins that implement onCreateNode for File nodes. These produce File children that are of their own type. E.g. markdownRemark, postsJson.

Gatsby stores these children in Redux as IDs in the parent’s children field. And then stores those child nodes as full Redux nodes themselves (see Node Creation for more). E.g. for a File node with two children, it will be stored in the Redux nodes namespace as:

An important note here is that we do not store a distinct collection of each type of child. Rather we store a single collection that they’re all packed into. The benefit of this is that we can create a File.children field that returns all children, regardless of type. The downside is that the creation of fields such as File.childMarkdownRemark and File.childrenPostsJson is more complicated. This is what createNodeFields does.

Another convenience Gatsby provides is the ability to query a node’s child or children, depending on whether a parent node has 1 or more children of that type.

child resolvers

When defining our parent File gqlType, createNodeFields will iterate over the distinct types of its children, and create their fields. Let’s say one of these child types is markdownRemark. Let’s assume there is only one markdownRemark child per File. Therefore, its field name is childMarkdownRemark. Now, we must create its GraphQL Resolver.

The resolve function will be called when we are running queries for our pages. A query might look like:

To resolve file.childMarkdownRemark, we take the node we’re resolving, and filter over all of its children until we find one of type markdownRemark, which is returned. Remember that children is a collection of IDs. So as part of this, we look up the node by ID in Redux too.

But before we return from the resolve function, remember that we might be running this query within the context of a page. If that’s the case, then whenever the node changes, the page will need to be rerendered. To record that fact, we call createPageDependency with the node ID and the page, which is a field in the context object in the resolve function signature.

parent field

When a node is created as a child of some node (parent), that fact is stored in the child’s parent field. The value of which is the ID of the parent. The GraphQL resolver for this field looks up the parent by that ID in Redux and returns it. It also creates a page dependency, to record that the page being queried depends on the parent node.

Plugin fields

These are fields created by plugins that implement the setFieldsOnGraphQLNodeType API. These plugins return full GraphQL Field declarations, complete with type and resolve functions.

File types

As described in plain object or value field, if a string field value looks like a file path, then we infer File as the field’s type. The creation of this type occurs in type-file.js setFileNodeRootType(). It is called just after we have created the GqlType for File (only called once).

It creates a new GraphQL Field Config whose type is the just created File GqlType, and whose resolver converts a string into a File object. Here’s how it works:

Say we have a data/posts.json file that has been sourced (of type File), and then the gatsby-transformer-json transformer creates a child node (of type PostsJson)

Notice that the image value looks like a file. Therefore, we’d like to query it as if it were a file, and get its relativePath, accessTime, etc.

The File type resolver takes care of this. It gets the value (images/BdiU-TTFP4h.jpg). It then looks up this node’s root NodeID via Node Tracking which returns the original data/posts.json file. It creates a new filename by concatenating the field value onto the parent node’s directory.

I.e data + images/BdiU-TTFP4h.jpg = data/images/BdiU-TTFP4h.jpg.

And then finally it searches Redux for the first File node whose path matches this one. This is our proper resolved node. We’re done!

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