[core] Skip system documents when exporting a dataset (#120)

packages/@sanity/core/src/commands/dataset/exportDatasetCommand.js

@@ -1,7 +1,9 @@
 import path from 'path'
 import fsp from 'fs-promise'
+import split from 'split2'
 import prettyMs from 'pretty-ms'
 import streamDataset from '../../actions/dataset/streamDataset'
+import skipSystemDocuments from '../../util/skipSystemDocuments'
 
 export default {
   name: 'export',
@@ -44,6 +46,8 @@ export default {
     const startTime = Date.now()
 
     streamDataset(client, dataset)
+      .pipe(split())
+      .pipe(skipSystemDocuments)
       .pipe(outputPath ? fsp.createWriteStream(outputPath) : process.stdout)
       .on('error', err => output.error(err))
       .on('close', () => {

packages/@sanity/core/src/util/skipSystemDocuments.js

@@ -0,0 +1,21 @@
+const through2 = require('through2')
+
+const parseJson = json => {
+  try {
+    return JSON.parse(json)
+  } catch (err) {
+    return null
+  }
+}
+
+const isSystemDocument = doc => doc && doc._id && doc._id.indexOf('_.') === 0
+
+module.exports = through2((line, enc, callback) => {
+  const doc = parseJson(line)
+
+  if (isSystemDocument(doc)) {
+    return callback()
+  }
+
+  return callback(null, `${line}\n`)
+})