Graphql query for optional field for childImageSharp

[Defite]

I have a blog with posts and previews for this posts. In my blog listing I query for this images like that:

edges {
				node {
					fields {
						slug
						langKey
					}
					frontmatter {
						date(formatString: "DD.MM.YY", locale: $langKey)
						excerpt
						title
						templateKey
						published
						path
						coverImg {
							childImageSharp {
								fluid(maxWidth: 700) {
									...GatsbyImageSharpFluid
								}
							}
						}
					}
				}
			}

And in my post I'm using fields like that:

title: Some title
...
coverImg: '/path/to/my/image.png'

All is working except case when none of my posts has coverImg field with valid path to the file in it. Gatsby throws an error:

There was an error in your GraphQL query:

Field "coverImg" must not have a selection since type "String" has no subfields.

This can happen if you e.g. accidentally added { } to the field "coverImg". If you didn't expect "coverImg" to be of type "String" make sure that your input source and/or plugin is correct.

I've related issues about Contentful, but didn't understand how to define properly type to make Graphql return null for coverImg field instead of this error.

Forgot to mention that I'm using gatsby theme as plugin, so this might be my issue: #15544 (comment)

I did some research and added this to my content site's gatsby-node.js:

exports.createSchemaCustomization = ({ actions }) => {
	const { createTypes } = actions;

	const typeDefs = [
	       `type MarkdownRemark implements Node {
			frontmatter: Frontmatter
		}`,
		`type Frontmatter @infer {
		         coverImg: [File!]! @fileByRelativePath,
		  }`,
	];

	createTypes(typeDefs);
};

And now Gatsby throws an error:

There was an error in your GraphQL query:

Cannot return null for non-nullable field Frontmatter.coverImg.

The field "Frontmatter.coverImg." was explicitly defined as non-nullable via the schema customization API (by yourself or a plugin/theme). This means that this field is not optional and you have to define a value. If this is not your desired behavior and you defined the schema yourself, go to "createTypes" in gatsby-node.js. If you're using a plugin/theme, you can learn more here on how to fix field types:
https://www.gatsbyjs.org/docs/schema-customization/#fixing-field-types

You can test it yourself in my theme repo: https://github.com/Defite/gatsby-theme-defite/tree/theme-v2

[lannonbr]

In your type definition in createSchemaCustomization, change the type of coverImg to just File. Having the [] around it say it's an array which I don't believe you want, and the ! means that field is mandatory, so having just File (and keeping the @fileByRelativePath extension) makes it completely optional which is what you want. Then in your markup you can just check for it existing by doing something like:

{node.frontmatter.coverImg && <Img ... />}

[Defite]

@lannonbr, yeah, just figured this out, thank you! Moving this code to gatsby-node.js to my site dir works just fine. And is there any way to get rid of this message:

warn Plugin `@defite/gatsby-theme-defite` tried to define the GraphQL type `MarkdownRemark`, which has already been defined by the plugin

Gatsby throw it if I change gatsby-node.js in gatsby-theme-defite.

[github-actions]

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[Defite]

Let it be hacks then...