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Hey Bruce, 
 
A Discriminator is term used in Object Oriented Programming, specifically with Single Table Inheritance (!). Here's a general overview. In Object code, when you're working with DBs, you'll often have a single table mapped to a single Object (Customers table maps to a Customer object). Sometimes however, your code and DB needs to &nbsp;deal with Inheritance - you may have a Worker model and DB table. You may want to extend (inheritance) that Worker model and have a Manager / Admin etc. These models are fundamentally based on a Worker. Now rather than having a separate table for Manager / Admin etc, you want to continue using the Worker table. But ... when you use the Worker table, you need to know whether you're working with a Worker, a Manager or an Admin. To do this, we add a new column to the Worker table that holds that vital piece of information - A Discriminator column. 
 
That's the explanation out of the way. In your case, you are dealing with an Activity. However, you can have different types of Activity (inheritance), and it identify what type of Activity you're dealing with, there is a Discriminator (call / email / event / visit etc). 
 
The error you're getting is telling you that somewhere, you're trying to create a Discriminator that already exists - i.e. Call. The discriminators needs to be unique (otherwise they couldn't possible work). 
 
I'm not exactly sure where you need to look in your code to find this, but it's likely to be close to the DB or Model layer, specifically the Activity model, so I would start there
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Hey Bruce, 
 
&quot;Uncaught ReferenceError: recordId is not defined&quot; means exactly what it says. In your code, you're running in strict mode (which is a good thing), but that does mean tighter rules on what you can and can't do. The error you're getting is because you're trying to assign a value to a variable that hasn't been declared (or it's not available in the current scope). You need to define it using either let (if it's going to change) or const (if it's not going to change). In the past, and with non-strict mode you could simply do this: 
 
myVar = &quot;Chris&quot; 
 
With strict mode, you need to declare the variable 
 
let myVar = &quot;Chris&quot; 
 
If your code is declaring it somewhere else, then it's likely to be a scope issues - variables are block scoped, so they're only available within the block they're declared in (functions for example).
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Hey! I changed this... recordId = companyId; to this... let recordId = companyId; And it worked. So, it was nothing more than just being a little more attentive to the fact that I'm working in a "strict" mode, and that's all it took?
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PHP Developer

Bruce Gust

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Chris, as a courtesy I'm going to close this question, but I just got word from my team lead that I'm going to have to resolve that Discriminator once and for all. He's got my pulling my branch down and making some edits. Without looking at it, I imagine I'll have the same troubles I had earlier today. That said, here are some questions that I'll have for tomorrow... <a href="https://www.experts-exchange.com/questions/29179915/What-is-Discriminator-and-Why-is-is-Problem-in-this-Case.html#questionAdd" rel="ugc">https://www.experts-exchange.com/questions/29179915/What-is-Discriminator-and-Why-is-is-Problem-in-this-Case.html#questionAdd</a>&nbsp; <a href="https://www.experts-exchange.com/questions/29179913/Is-this-the-right-sequence-of-Git-commands-for-this-scenario.html" rel="ugc">https://www.experts-exchange.com/questions/29179913/Is-this-the-right-sequence-of-Git-commands-for-this-scenario.html</a>&nbsp;
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I'm running into an error that I don't know how to fix: 
 
error&quot;:true,&quot;msg&quot;:&quot;Discriminator with name \&quot;call\&quot; already exists&quot;,&quot;data&quot;:[]} 
 
The route for this is &quot;add-activity&quot; which is going to save a record to the &quot;activities&quot; collection. Here's the service / schema for that model: 
 
const mongoose = require('mongoose'); 
const Service = require('./service'); 
const ActivitySchema = require('../schemas/ActivitySchema'); 
 
class Activity extends Service { 
&nbsp; constructor() { 
&nbsp; &nbsp; super(); 
 
&nbsp; &nbsp; this.collection = 'activities'; 
&nbsp; &nbsp; this.plural = 'activities'; 
&nbsp; &nbsp; this.singular = 'activity'; 
&nbsp; &nbsp; this.model = mongoose.model('Activity', ActivitySchema); 
&nbsp; } 
} 
 
module.exports = new Activity(); 
 
I've got no idea what to be looking for. No doubt this error has been encountered before and I'm furiously hitting up Google even as you're perusing this question. 
 
What does it mean and how do I fix it?
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I'm running into an error that I don't know how to fix:

error":true,"msg":"Discriminator with name \"call\" already exists","data":[]}

The route for this is "add-activity" which is going to save a record to the "activities" collection. Here's the service / schema for that model:

const mongoose = require('mongoose');
const Service = require('./service');
const ActivitySchema = require('../schemas/ActivitySchema');

class Activity extends Service {
  constructor() {
    super();

    this.collection = 'activities';
    this.plural = 'activities';
    this.singular = 'activity';
    this.model = mongoose.model('Activity', ActivitySchema);
  }
}

module.exports = new Activity();

I've got no idea what to be looking for. No doubt this error has been encountered before and I'm furiously hitting up Google even as you're perusing this question.

What does it mean and how do I fix it?

Discriminator with name \&quot;call\&quot; already exists...?

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Discriminator with name \&quot;call\&quot; already exists...?

Discriminator with name \"call\" already exists...?