const person = realm.objectForPrimaryKey('Person', id);
console.log(person.addListener != null);

// App.js - Created from an ejected expo app
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import Realm from 'realm';

const DogSchema = {
  name: 'Dog',
  primaryKey: 'id',
  properties: {
    id: 'int',
    name: 'string',
  },
}
class Cat { }
Cat.schema = {
  name: 'Cat',
  primaryKey: 'id',
  properties: {
    id: 'int',
    name: 'string',
  },
}

const schema = [
  DogSchema, // This works
  Cat        // This doesn't
]

export default class App extends React.Component {

  constructor(props) {
    super(props);
    this.state = {
      realm: null,
      dog: undefined,
      cat: undefined,
    };
  }

  componentWillMount() {
    Realm.open({ schema }).then(realm => {
      realm.write(() => {
        realm.create('Dog', { name: 'Rex', id: 0 }, true);
        realm.create('Cat', { name: 'Tilly', id: 0 }, true);
      });
      this.setState({ 
        realm,
        dog: realm.objectForPrimaryKey('Dog', 0),
        cat: realm.objectForPrimaryKey('Cat', 0),
      });
    });
  }

  render() {
    let dogInfo = 'Loading...';
    let catInfo = '';
    if (this.state.realm) {
      dogInfo = `dog.addListener exist? ${this.state.dog.addListener != null}`;
      catInfo = `cat.addListener exist? ${this.state.cat.addListener != null}`;
    }

    return (
      <View style={styles.container}>
        <Text>{dogInfo}</Text>
        <Text>{catInfo}</Text>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    alignItems: 'center',
    justifyContent: 'center',
  },
});