title: Appsync & GraphQL: how to filter a list by nested value
body: <p>I have an Appsync API generated by Amplify from a basic schema. On the <code>Article</code> model, a <code>category</code> field is nested within a <code>metadata</code> field. I want to build a Query that provides a list of Articles filtered by category. It is not clear to me how to filter on a nested value... I have seen <a href="https://stackoverflow.com/questions/48532299/gatsby-js-filter-graphql-query-by-nested-object-property">similar questions</a> but the analogous answer has not worked.</p>
  
  <p><strong>AWS GraphQL Transform Schema</strong></p>
  
  <pre><code>type Article @model {
    id: ID!
    title: String!
    description: String!
    text: String!
    metadata: ArticleMetadata!
  }
  
  type ArticleMetadata {
    category: Category!
    lastModified: String!
    creationDate: String!
  }
  
  enum Category {
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    java
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    python
    haskell
  }
  </code></pre>
  
  <p><strong>Generated List Query</strong></p>
  
  <pre><code>export const listArticles = `query ListArticles(
    $filter: ModelArticleFilterInput
    $limit: Int
    $nextToken: String
  ) {
    listArticles(filter: $filter, limit: $limit, nextToken: $nextToken) {
      items {
        id
        title
        description
        text
        metadata {
          category
          lastModified
          creationDate
        }
      }
      nextToken
    }
  }
  `;
  </code></pre>
  
  <p><strong>Failing filter query</strong></p>
  
  <pre><code>query listArticlesByCategory($category: String!) {
    listArticles(filter: {category: {eq: $category}}) { 
      items {
        title
        description
        text
        metadata {
          category
          creationDate
          lastModified
        }
      }
    }
  }
  </code></pre>
  
  <p>The Appsync console error states that the <code>category</code> in <code>filter: {category: ... }</code> is an unknown field.</p>