Unable to use SelectMany to fetch parent property in DocumentDb LINQ query

Question

I have a nested document structure like:

[
  {
    "id": "parent1",
    "children": [
      {
        "id": "child1",
        "foo": "bar"
      },
      {
        "id": "child2",
        "foo": "bar"
      },
      {
        "id": "child3",
        "foo": "bar"
      }
    ]
  },
  {
    "id": "parent2",
    "children": [
      {
        "id": "child4",
        "foo": "bar"
      },
      {
        "id": "child5",
        "foo": "bar"
      }
    ]
  }
]

I am able to write the following query in SQL syntax:

SELECT child, parent.id
FROM parent
JOIN child in parent.children

This gets me the following result:

[
  {
    "child": {
      "id": "child1",
      "foo": "bar"
    },
    "id": "parent1"
  },
 ...
]

I wrote a similar query in LINQ using SelectMany clause as follows, but it throws an error stating SelectMany can have only 2 arguments.

collection.SelectMany(
    parent => parent.children,
    (parent, child) => new { child, parent.id });

Your query looks quite OK to me. Is it the O-R mapper that doesn't understand this SelectMany overload? Exact error message? In case of yes, the answer of dasblinkenlight could solve it. — tinudu
– tinudu, Commented Aug 7, 2017 at 14:40

Sergey Kalinichenko · Accepted Answer · 2017-08-07 13:56:20Z

4

You need to "push" the second lambda inside the first lambda with a nested Select, like this:

collection.SelectMany(
    parent => parent.children.Select(child => new {
        Child = child
    ,   ParentId = parent.id
    })
);

answered Aug 7, 2017 at 13:56

Sergey Kalinichenko

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1 Comment

Shaun Whyte Over a year ago

This is one of those situations where you google something for a long period of time and then stumble upon the answer almost randomly

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