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Accessing nested union type content in Sanity preview select

3 replies
Last updated: Nov 29, 2025
I have a schema that looks like below.
The issue is that I want to show the thumbnail/title from deeply nested objects and I'm not sure how to achieve that. 
content
can be either 
photo
or 
video
and they have slightly different structure. But it seems like I need to use dot notation in the 
select
to be able to get the reference properly, does that mean I should do something like:

select: {
  videoContent: 'content.[0].video',
  imageContent: 'content.[0].image.asset.originalFilename'
  layout: 'layout',
  bleed: 'bleed',
},
and then use 
prepare
to determine if it's if type photo or video? Feels like there must be a better way.

production.js:
export default defineType({
  name: 'production',
  title: 'Productions',
  type: 'document',
  fields: [
    defineField({
      title: 'Content',
      name: 'content',
      type: 'array',
      of: [{
        type: 'productionContent',
      }]
    }),
  ],
})

productionContent.js:
export default {
  name: 'productionContent',
  title: 'Content',
  type: 'object',
  fields: [{
    name: 'content',
    title: 'Content',
    type: 'content',
  }],
  preview: {
    select: {
      content: 'content.[0]', <-- how to get either video or photo content here?
    },
    prepare(selection) {
      return {
        ...selection,
      }
    }
  }
}

content.js:
export default {
  name: 'content',
  title: 'Content',
  type: 'array',
  of: [
    {
      title: 'Photo',
      name: 'photo',
      type: 'photo',
    },
    {
      title: 'Video',
      name: 'video',
      type: 'video',
    },
  ]
}

photo.js
export default {
  name: 'photo',
  title: 'Photo',
  type: 'object',
  fields: [
    {
      title: 'Photo',
      name: 'image',
      type: 'image',
    },
  ],
  preview: {
    select: {
      media: 'image',
      title: 'image.asset.originalFilename'
    },
    prepare(selection) {
      return {
        ...selection,
        status: 'Photo'
      }
    }
  },
}

video.js
export default {
  name: 'video',
  title: 'Video',
  type: 'object',
  fields: [
    {
      title: 'Title',
      name: 'title',
      type: 'string',
    },
    {
      title: 'Video',
      name: 'video',
      type: 'mux.video',
    },
    {
      title: 'Preview Image',
      name: 'image',
      type: 'image',
    }
  ],
  preview: {
    select: {
      title: 'title',
      media: 'image'
    },
    prepare(selection) {
      return {
        ...selection,
        status: 'Video', 
      }
    }
  },
}
AI Update

Looking at your schema, you're dealing with nested arrays and union types, which makes preview selection tricky. The good news is that Sanity's select in preview configuration supports accessing nested properties, and you can handle the union type logic in the prepare function.

Your intuition is correct - you'll need to select the fields you need using dot notation and then determine the type in prepare. Here's the recommended approach:

// productionContent.js
export default {
  name: 'productionContent',
  title: 'Content',
  type: 'object',
  fields: [{
    name: 'content',
    title: 'Content',
    type: 'content',
  }],
  preview: {
    select: {
      // Select the first item's type to determine photo vs video
      contentType: 'content.0._type',
      // Select photo fields
      photoImage: 'content.0.image',
      photoFilename: 'content.0.image.asset.originalFilename',
      // Select video fields
      videoTitle: 'content.0.title',
      videoImage: 'content.0.image',
    },
    prepare(selection) {
      const { contentType, photoImage, photoFilename, videoTitle, videoImage } = selection;
      
      if (contentType === 'photo') {
        return {
          title: photoFilename || 'Untitled Photo',
          subtitle: 'Photo',
          media: photoImage,
        }
      }
      
      if (contentType === 'video') {
        return {
          title: videoTitle || 'Untitled Video',
          subtitle: 'Video',
          media: videoImage,
        }
      }
      
      return {
        title: 'No content',
      }
    }
  }
}

The key points:

  1. Use content.0._type to determine which union type you're dealing with. Every Sanity object has a _type field that identifies its schema type.

  2. Select all possible fields you might need from both photo and video types. The ones that don't exist for the actual type will just be undefined.

  3. Branch in prepare based on the _type to return the appropriate title, subtitle, and media.

This is indeed the standard pattern for handling union types in preview configurations. While it feels verbose, it's the most reliable way to handle polymorphic content in Sanity previews.

Alternative consideration: If your content array typically only has one item (which seems to be the case from your schema structure), you might want to reconsider if you need the array wrapper at all. If productionContent.content is always a single item, you could make it a direct union type instead of an array, which would simplify your preview configuration to just content._type, content.image, etc. without the array indexing.

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