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How to get and set a ref for a newly cached related object in Apollo client InMemoryCache?

I have a set of related items like so:
book {
id
...
related_entity {
id
...
}
}
which apollo caches as two separate cache objects, where the related_entity field on book is a ref to an EntityNode object. This is fine, the related entity data is also used elsewhere outside of the context of a book so having it separate works, and everything seems well and good and updates as expected...except in the case where the related entity does not exist on the initial fetch (and thus the ref on the book object is null) and I create one later on.
I've tried adding an update function to the useMutation hook that creates the aforementioned related_entity per their documentation: https://www.apollographql.com/docs/react/caching/cache-interaction/#example-adding-an-item-to-a-list like this:
const [mutateEntity, _i] = useMutation(CREATE_OR_UPDATE_ENTITY,{
update(cache, {data}) {
cache.modify({
id: `BookNode:${bookId}`,
fields: {
relatedEntity(_i) {
const newEntityRef = cache.writeFragment({
fragment: gql`
fragment NewEntity on EntityNode {
id
...someOtherAttr
}`,
data: data.entityData
});
return newEntityRef;
}
}
})
}
});
but no matter what I seem to try, newEntityRef is always undefined, even though the new EntityNode is definitely in the cache and can be read just fine using the exact same fragment. I could give up and just force a refetch of the Book object, but the data is already right there.
Am I doing something wrong/is there a better way?
Barring that is there another way to get a ref for a cached object given you have its identifier?

It looks like this is actually an issue with apollo-cache-persist - I removed it and the code above functions as expected per the docs. It also looks like I could instead update to the new version under a different package name apollo3-cache-persist, but I ended up not needing cache persistence anyway.

adding a new property to existing doesn't work

I'm trying to add another property to existing object returning from findOne() promise in mongoose.
In the response I get the object without the property convertName
app.get('/getItem', (req, res) => {
var itemID = req.query.itemID;
Item.findOne({_id: itemID}).then(item => {
item.convertName = 'cm';
res.send(item);
}).catch( err => {
res.status(401).send();
});
})
I know that the way to add another property to an existing object is similar to this, just specify the property name and set a value to it, so I don't know why it is not working in this case.
Hope you can explain and help me why its not working.

It's a bit complicated with Mongoose: by default MongooseDocument is returned by query, and the property that you try to add to such a document is not reflected on its serialized value (the one that is sent in the response).
One possible way around this is using lean() method to enable lean option. Quoting the doc:
Documents returned from queries with the lean option enabled are plain
javascript objects, not MongooseDocuments. They have no save method,
getters/setters or other Mongoose magic applied.

Stripped Attributes with toJSON/toObject

I have something like the following code:
User.findOne(id)
.exec((err, user) => {
Pets.find(_.pluck(user.pets, 'id'))
.populate("toys")
.exec((err, petsWithToys) => {
user.pets = petsWithToys;
return res.ok({ user: user });
});
});
When I look at the response in the client I don't see the toys array inside the pet.
I thought maybe this was due to overriding the toJSON function in my User model but even when removing it I get the same behavior.
Also, I've found out that if I assign the values to a new property that is not defined in the model, I do see the values at the client. I.e. if I do
user.petsNew = petsWithToys;
I will see the fully populated property.
I've seen the documentation of toObject where is says it removes instance methods (here) but I am not sure why the collection is considered a method and don't understand how after changing the value it is still removed.
Any comments/explanations/workarounds?
P.S. Tried to step through the code but can't step into toObject...

Add user = user.toJSON(); before user.pets = petsWithToys;
Check https://stackoverflow.com/a/43500017/1435132

Passing array via ReactiveVar in Meteor

I have a Meteor method that returns all user accounts on my application
returnUsers: function(){
return Meteor.users.find().fetch();
}
I'm using new ReactiveVar to pass the return value of the Meteor method into my template helper:
Template.listViewTemplate.created = function (){
var self = this;
self.myAsyncValue = new ReactiveVar("Waiting for response from serv er...");
Meteor.call('returnUsers', function (err, users) {
if (err)
console.log(err);
else
self.myAsyncValue.set(users);
});
}
Template.listViewTemplate.helpers({
userCollection: function(){
return Template.instance().myAsyncValue.get();
}
});
But when I go to render the users into the view, I get a console error that reads
{{#each}} currently only accepts arrays
When I render without the #each iterator, using
<ul id='usersList'>
{{userCollection}}
</ul>
the output on my web-page accurately reflects the number of users (2), but reads
[object Object],[object Object]
I'm pretty sure that there is some funkiness going on here because I'm using a global Meteor collection (Meteor.users.find().fetch(), as opposed to having defined my own collection), but I'm not sure how to get around it.
I want to display a list of all users so the current user can click another user and share a document with them--not sure how to get around this.

You don't need to use a reactive variable for this. The function at Template.listViewTemplate.created is not container in an autorun, which means: It won't get recomputed.
The best approach for your scenario is: Use a variable to get the status ( loading, loaded, error) and another variable to save the array itself attach to self. Reactivity is cool but you should only use it when needed.
About:
[object Object],[object Object]
This is happening because you're not extracting any value form the object provided nor looping using {{#each}}.
Your solutions for listing users is dangerous and inefficient. You're sending to the client all the fields from the user collection, including login tokens.
The best approach is to create a subscription that send only the necessaries fields like: _id, info.firstName. You should also have some criteria to the list users and use pagination. Consider also a search feature for such purpose.

ReactiveVar doesn't like arrays. You could install the ReactiveArray package which should accomplish exactly what you want.
Update
Based on comment of mper
In the latest versions of Meteor you can put an array in a ReactiveVar.
Tested on
meteor#1.6.0
reactive-var#1.0.11

I have several remarks about your question:
Do not fetch
You don't need .fetch() on your method. When you call find() on collections, such as Meteor.users a cursor is returned. The template (and #each in particular) can iterate through cursors. Cursors are usually better because you don't load the entire collection into memory at once - fetch does.
Meteor collections are reactive
Meteor collections are already reactive, meaning that if they change, they will trigger changes on your templates as well. So, you don't need to use a ReactiveVar to wrap your collection.
Query your local database
You don't need to use a method to get the users and in fact, you shouldn't, because usually you want to make queries to the database stored locally, not make calls to the server. Just call Meteor.users.find() directly in your template helper. You can (and should) control what is available locally through subscriptions.
Use #each with else
You can use the following in your template:
{{#each userCollection}}
...
{{else}}
Waiting for response from server...
{{/each}}
If userCollection is empty, the template will render the else block, just like you wanted.
Summarizing
Delete your method and onCreated with everything inside, change whatever is inside your template helper to only return Meteor.users.find() and use {{#each userCollection}}...{{else}}Waiting for response from server...{{/else}}
By the way
In the latest versions of Meteor you can put an array in a ReactiveVar.

Template.onCreated(function(){}) only gets run once and meteor methods only run once
You need reactivity here.
Collections sre reactive meaning pub/sub.
You need to create a publish function that allows certain users to fetch other users in the database. So all uses with maybe if the currentUser has permission to read all user info. Id limit the fields too.

breezejs: adding referential constraint to an entity type

This is a follow-up question to my previous issue - this one was getting a bit messy and is more related to the Telerik Data Service.
The metadata I receive from the server are missing the referential constraints in the association node, although I've set the foreign key attribute on my model.
Therefore I was thinking about manually adding these constraints to my entities in the callback of FetchMetadata.
Is that possible and can someone provide a simple example on how to do it ?
[EDIT]
Here's what I have so far:
manager.fetchMetadata().then(function () {
var mandatType = manager.metadataStore.getEntityType("Mandate");
mandatType.autogeneratedKeyType = breeze.AutoGeneratedKeyType.Identity;
var openPositionsProp = new breeze.NavigationProperty({
name: "OpenPositions",
entityTypeName: "OpenPositions:#DirectDebitModel",
isScalar: true,
associationName: "OpenPosition_Mandate_Mandate_OpenPositions",
foreignKeyNames: ["Id"]
});
mandatType.addProperty(openPositionsProp);
});
But it raises the exception:
The 'Mandate:#DirectDebitModel' EntityType has already been added to a MetadataStore and therefore no additional properties may be added to it.

Ok, I have a possible approach that you might be able to use right now.
Fetch the metadata from Teleriks OData feed just like you do now.
Export the metadataStore created as a result of the previous step via the MetadataStore.exportMetadata method. This will return "stringified" json for the same metadata in Breeze's native format. This format is much easier to work with.
Convert this string to json via JSON.parse.
Modify the json to add referential constraint information. See Breeze Native Metadata format docs here
Create a new MetadataStore and import the modified json into it.
Create a new EntityManager with this MetadataStore and use it. This EntityManager should now have complete Breeze metadata for use with the rest of the feed.
Hope this makes sense!
We are planning on releasing a form of hybrid metadata in the next release. Unfortunately, it doesn't cover your case because we are focusing on how to add custom metadata to an existing metadataStore, and not actually edit/modify the existing metadata.
Another alternative is that we (IdeaBlade) do offer consulting for this type of work. We could probably write a tool that does steps 1 thru 6 for you. Please contact breeze#ideablade.com if this is of interest and mention this post.

So you are getting meta data but it doesn't have a relationship between the entities. Hmm I have not gotten metaData AND tried to create additional model properties that are related.
Your best bet is to add a property that is a navigation type on the constructor.
http://www.breezejs.com/sites/all/apidocs/classes/EntityType.html#method_addProperty
If it were me, I would try it this way (or something similar) inside of the constructor -
myEntity.addProperty({
associatedEntity: {
entityTypeName: "AssociatedEntity", isScalar: true,
associationName: "AssociatedEntity_MyEntitys", foreignKeyNames: ["associatedEntityId"]
}
});
Where myEntity is the name of the current entity, AssociatedEntity would be your navigation property, the associatedEntityId is a property of myEntity that refers to the other entity. Of course to have this be a two-way relationship you would need to add a property to AssociatedEntity as well.
associatedEntity.addProperty({
myEntitys: {
entityTypeName: "MyEntity", isScalar: true,
associationName: "AssociatedEntity_MyEntitys", foreignKeyNames: ["myEntityId"]
}
});