How to create a full-text index using properties of subclasses in MongoDB

Question

Let's say I have an abstract class Animal, and subclasses Dog and Cat.

My MongoDB table stores all animals, and uses a the _t type discriminator field in documents.

What I want to do is create a full-text index on 2 properties, one that exists only in Dog documents, and one that exists only in Cat documents. The JSON is straightfoward enough:

{ 
    "dog_only_field" : "text", 
    "cat_only_field" : "text"
}

But I'd like to do this with the official C# driver. I tried this:

var builder = Builders<Animal>.IndexKeys;
var keys = builder.Text(x => ((Dog)x).MyText).Text(x => ((Cat)x).MyText);
col.Indexes.CreateOne(keys);

But got Unable to determine the serialization information for x => Convert(x).Message.

How does your Cat, Dog and Animal classes look like? Please post bare minimum testable code. — Saleem
– Saleem, Commented Mar 31, 2016 at 14:01
@Saleem TBH, I'm not sure it adds anything useful to understanding the question, but let's say that Animal only has a Guid Id property, Dog has a string property DogText and Cat has a string property CatText. — Cocowalla
– Cocowalla, Commented Mar 31, 2016 at 14:12
It's not about understanding but testing also. Problem may be crystal clear to you but think about others who have no clue of your thought process. — Saleem
– Saleem, Commented Mar 31, 2016 at 14:15

Cocowalla · Accepted Answer · 2016-03-31 14:31:30Z

1

I got this working by using field definitions that specify the JSON property name as a string:

var dogFieldDef = new StringFieldDefinition<Animal>("dog_only_field");
var catFieldDef = new StringFieldDefinition<Animal>("cat_only_field");

var builder = Builders<Animal>.IndexKeys;

var key = builder
    .Text(dogFieldDef)
    .Text(catFieldDef);

col.Indexes.CreateOne(key);

answered Mar 31, 2016 at 14:31

Cocowalla

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