Achieving this in Gatsby used to be pretty troublesome, but thanks to the new createSchemaCustomization Node API docs (since Gatsby 2.5) it's relatively easy.
Here's a demo where I replicate your repo structure: github
Here's where the relevant code lives: github
Here's the code to make it work:
// gatsby-node.js
const path = require('path')
exports.createSchemaCustomization = ({ actions }) => {
const { createFieldExtension, createTypes } = actions
createFieldExtension({
name: 'fileByDataPath',
extend: () => ({
resolve: function (src, args, context, info) {
const partialPath = src.featureImage
if (!partialPath) {
return null
}
const filePath = path.join(__dirname, 'src/data', partialPath)
const fileNode = context.nodeModel.runQuery({
firstOnly: true,
type: 'File',
query: {
filter: {
absolutePath: {
eq: filePath
}
}
}
})
if (!fileNode) {
return null
}
return fileNode
}
})
})
const typeDefs = `
type Frontmatter @infer {
featureImage: File @fileByDataPath
}
type MarkdownRemark implements Node @infer {
frontmatter: Frontmatter
}
`
createTypes(typeDefs)
}
How it works:
There are 2 parts to this:
- Extend
markdownRemark.frontmatter.featureImage so graphql resolves to a File node instead of a string via createTypes
- Create a new field extension
@fileByDataPath via createFieldExtension
createTypes
Right now Gatsby's inferring frontmatter.featureImage as a string. We'll ask Gatsby to read featureImage as a string instead, by modifying its parent type:
type Frontmatter {
featureImage: File
}
This is not enough however, we'll also need to pass this Frontmatter type to its parent as well:
type Frontmatter {
featureImage: File
}
type MarkdownRemark implements Node {
frontmatter: Frontmatter
}
We'll also add the @infer tag, which lets Gatsby know that it can infer other fields of these types, i.e frontmatter.title, markdownRemark.html, etc.
Then pass these custom type to createTypes:
exports.createSchemaCustomization = ({ actions }) => {
const { createTypes } = actions
const typeDefs = `
type Frontmatter @infer {
featureImage: File
}
type MarkdownRemark implements Node @infer {
frontmatter: Frontmatter
}
`
createTypes(typeDefs)
}
Now, we can fire up localhost:8000/___graphql and try to query the image
query Post {
markdownRemark {
frontmatter {
featureImage {
id
}
}
}
}
and we get...
Error: Cannot return null for non-nullable field File.id.
That is because while Gatsby now understands featureImage should be a File node, it has no idea where to get that file.
At this point, we can either use createResolvers to manually resolve the field to a File node, or createFileExtension to do the same thing. I choose createFileExtension because it allows more code reuse (you can extend any fields), while createResolvers, in this case, is more useful for a specific field. Seeing that all you want is to resolve a file from the src/data directory, I'll call this extension fieldByDataPath.
createFileExtension
Let's just look at the resolve attribute. It is a function that takes in the following:
- source: The data of the parent field (in this case,
frontmatter)
- args: The arguments passed to
featureImage in a query. We won't need this
- context: contains
nodeModel, which we'll use to get nodes from Gatsby node store
- info: metadata about this field + the whole schema
We will find the original path (img/photo.jpg) from src.featureImage, then glue it to src/data to get a complete absolute path. Next, we query the nodeModel to find a File node with the matching absolute path. Since you have already pointed gatsby-source-filesystem to src/data, the image (photo.jpg) will be in Gatsby node store.
In case we can't find a path or a matching node, return null.
resolve: async function (src, args, context) {
// look up original string, i.e img/photo.jpg
const partialPath = src.featureImage
if (!partialPath) {
return null
}
// get the absolute path of the image file in the filesystem
const filePath = path.join(__dirname, 'src/data', partialPath)
// look for a node with matching path
const fileNode = await context.nodeModel.runQuery({
firstOnly: true,
type: 'File',
query: {
filter: {
absolutePath: {
eq: filePath
}
}
}
})
// no node? return
if (!fileNode) {
return null
}
// else return the node
return fileNode
}
We've done 99% of the work. The last thing to do is to move this to pass this resolve function to createFieldExtension; as well as add the new extension to createTypes
createFieldExtension({
name: 'fileByDataPath' // we'll use it in createTypes as `@fileByDataPath`
extend: () => ({
resolve, // the resolve function above
})
})
const typeDef = `
type Frontmatter @infer {
featureImage: File @fileByDataPath // <---
}
...
`
With that, you can now use relative path from src/data/ in frontmatter.
Extra
The way fileByDataPath implemented, it'll only work with fields named featureImage. That's not too useful, so we should modify it so that it'll work on any field that, say, whose name ended in _data; or at the very least accept a list of field names to work on.
Edit had a bit of time on my hand, so I wrote a plugin that does this & also wrote a blog on it.
Edit 2 Gatsby has since made runQuery asynchronous (Jul 2020), updated the answer to reflect this.
