Projecting Results

A specification gives you the facts that match a set of conditions. But sometimes you want to transform the facts into a different shape. You can do that with the select method.

const postsInSite = model.given(Site).match((site, facts) =>
  facts.ofType(Post)
    .join(post => post.site, site)
    .select(post => ({
      hash: j.hash(post),
      createdAt: post.createdAt
    }))
);

When you select the hash or a field of a fact, the resulting value is immutable. That's because facts themselves are immutable. If you want to project something that changes, you can select a sub-specification.

const postsInSite = model.given(Site).match((site, facts) =>
  facts.ofType(Post)
    .join(post => post.site, site)
    .select(post => ({
      hash: j.hash(post),
      createdAt: post.createdAt,
      titles: facts.ofType(PostTitle)
        .join(title => title.post, post)
        .select(title => title.value)
    }))
);

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