How to query Solr for child documents

Question

I'm trying to run a block join query to pull child documents but I'm getting a "Parent query yields document which is not matched by parents filter" error. I'm using Solr 5.5

My schema looks like this:

<field name="url" type="string" indexed="true" stored="true" required="true" multiValued="false" />
<field name="name" type="text_en" indexed="true" stored="true" required="true" multiValued="false" />
<field name="content_type" type="string" indexed="true" stored="true" required="true" multiValued="false" />
<uniqueKey>url</uniqueKey>

Insert statement looks like this:

{
  "url": "http://www.test.com/index.html",
  "name": "parent product",
  "content_type": "parentDocument",
  "_childDocuments_": [
    {
      "url": "http://www.test.com/index2.html",
      "name": "child product",
      "content_type": "childDocument"
    },
    {
      "url": "http://www.test.com/index3.html",
      "name": "child product 2",
      "content_type": "childDocument"
    }
  ]
}

Running a standard *:* query in the console pulls back the 3 documents and shows the children belong to their parent:

{
  "docs": [
    {
      "url": "http://www.test.com/index2.html",
      "name": "child product",
      "content_type": "childDocument",
      "_root_": "http://www.test.com/index.html"
    },
    {
      "url": "http://www.test.com/index3.html",
      "name": "child product 2",
      "content_type": "childDocument",
      "_root_": "http://www.test.com/index.html"
    },
    {
      "url": "http://www.test.com/index.html",
      "name": "test product",
      "content_type": "parentDocument",
      "_version_": 1541193313504198700,
      "_root_": "http://www.test.com/index.html"
    }
  ]
}

However If I run q={!child of="content_type:parentDocument"} I get the parent document which I would not expect given the "child of" statement:

{
  "responseHeader": {
    "status": 0,
    "QTime": 0,
    "params": {
      "q": "{!child of=\"content_type:parentDocument\"}",
      "indent": "true",
      "wt": "json"
    }
  },
  "response": {
    "numFound": 1,
    "start": 0,
    "docs": [
      {
        "url": "http://www.test.com/index.html",
        "name": "test product",
        "content_type": "parentDocument",
        "_version_": 1541193313504198656,
        "_root_": "http://www.test.com/index.html"
      }
    ]
  }
}

But if I add any sort of query I get an error eg

q={!child of="content_type:parentDocument"}name:product 

or even

q={!child of="content_type:parentDocument"}name:*

"Parent query yields document which is not matched by parents filter, docID=0"

Answer 1

So as I now understand it the query cannot return child documents. It's almost like a query and a filter. The query ie name:* could match parent and child documents which is not allowed. I added an additional filter ie +name:product +content_type:parentDocument to restrict results to only parents. I then added the {!child of="content_type:parentDocument"} to get the children of these parents so I now have:

{!child of="content_type:parentDocument"}+name:product +content_type:parentDocument

and this works as expected.

Similarly the inverse would be:

{!parent which="content_type:parentDocument"}+name:product +content_type:childDocument

To get the parents of children with name:product