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Generate Firestore document's doc Id based on Users' uids

In my chat app, I have private chat between the two users. I intend to set the chat document's id using these two user's docId/uid in such a way that it doesn't depend on the order they're combined and I can determine the chat document's docId using the uid of users irrespective of the order of uid.
I know, I can use where clauses to get the chat doc as well. Is there any major flaw with my approach of generating the chat document's docId? Should I let it be generated automatically and use normal where clauses supported by firestore and limit(1) to get the chat?
basically, it seems I'm looking for is to encrypt uid1 in such a way that it returns a number only and then same with uid2 and then add them together to create the ChatId. This way it'll not depend on the order I use to add them and I can get the chatId and maybe convert that number back to a string using Base64 encode. This way, if I know the users participating in the chat, I can generate the same ChatId. Will that work or is there any flaw to it?

Converting each user ID to a number and then adding them together will likely lead to collisions. As a simple example, think of the many ways you can add up to the number 5: 0+5, 1+4, 2+3.
This answer builds upon #NimnaPerera's answer.
Method 1: <uid>_<uid>
If your app doesn't plan on using large groups, you can make use of the <uid>_<uid> format. To make sure the two user IDs are ordered in the same way, you can sort them first and then combine them together using some delimiter.
A short way to achieve this is to use:
const docId = [uid1, uid2].sort().join("_");
If you wanted to have a three-way group chat, you'd just add the new userID in the array:
const docId = [uid1, uid2, uid3].sort().join("_");
You could also turn this into a method for readability:
function getChatIdForMembers(userIds) {
return userIds.sort().join("_");
}
Here's an example of it in action:
const uid1 = "apple";
const uid2 = "banana";
const uid3 = "carrot";
[uid1, uid2].sort().join("_"); // returns "apple_banana"
[uid1, uid3].sort().join("_"); // returns "apple_carrot"
[uid2, uid1].sort().join("_"); // returns "apple_banana"
[uid2, uid3].sort().join("_"); // returns "banana_carrot"
[uid3, uid1].sort().join("_"); // returns "apple_carrot"
[uid3, uid2].sort().join("_"); // returns "banana_carrot"
// chats to yourself are permitted
[uid1, uid1].sort().join("_"); // returns "apple_apple"
[uid2, uid2].sort().join("_"); // returns "banana_banana"
[uid3, uid3].sort().join("_"); // returns "carrot_carrot"
// three way chat
[uid1, uid2, uid3].sort().join("_"); // returns "apple_banana_carrot"
[uid1, uid3, uid2].sort().join("_"); // returns "apple_banana_carrot"
[uid2, uid1, uid3].sort().join("_"); // returns "apple_banana_carrot"
[uid2, uid3, uid1].sort().join("_"); // returns "apple_banana_carrot"
[uid3, uid1, uid2].sort().join("_"); // returns "apple_banana_carrot"
[uid3, uid2, uid1].sort().join("_"); // returns "apple_banana_carrot"
Method 2: Member list properties
If you intend on supporting group chats, you should use automatic document IDs (see CollectionReference#add()) and store a list of chat members as one of it's fields as introduced in #NimnaPerera's answer for better use of queries.
I recommend two fields:
"members" - an array containing each chat member's ID. This allows you to query the /chats collection for chats that contain the given user.
"membersAsString" - a string, built from sorting "members" and joining them using "_". This allows you to query the /chats collection for chats that contain the exact list of members.
"chats/{chatId}": {
"members": string[], // list of users in this chat
"membersAsString": string, // sorted list of users in this chat, delimited using "_"
/* ... */
}
To find all chats that I am a part of:
const myUserId = firebase.auth().currentUser.uid;
const myChatsQuery = firebase.firestore()
.collection("chats")
.where("members", "array-contains", myUserId);
myChatsQuery.onSnapshot(querySnapshot => {
// do something with list of chat documents
});
To find all three-way chats between Apple, Banana and I:
const myUserId = firebase.auth().currentUser.uid;
const members = [myUserId, "banana", "apple"];
const membersAsString = members.sort().join("_");
const groupChatsQuery = firebase.firestore()
.collection("chats")
.where("membersAsString", "==", membersAsString);
groupChatsQuery.onSnapshot(querySnapshot => {
// do something with list of chat documents
// normally this would return 1 result, but you may get
// more than one result if a user gets added/removed a chat
});
A normal flow, would be to:
Get a list of the relevant chats
For each chat, get the most recent message
Based on the most recent message, sort the chats in your UI

You can very well use a combination of two users uids to define your Firestore document IDs, as soon as you respect the following constraints:
Must be valid UTF-8 characters
Must be no longer than 1,500 bytes
Cannot contain a forward slash (/)
Cannot solely consist of a single period (.) or double periods (..)
Cannot match the regular expression __.*__
What I'm not sure to understand in your question is "in such a way that it doesn't depend on the order they're combined". If you combine the uids of two users you need to combine them in a certain order. For example, uid1_uid2 is not equal to ui2_uid1.

As you are asking #lightsaber you can follow following methods to achieve your objective. But my personal preference is using an where clause, because firestore is supporting that compound queries which cannot be done in real time database.
Method 1
Create a support function to generate a chatId and check whether document is exist from that id. Then you can create chat document or retrieve the document using that id.
const getChatId = (currentUserId: string, guestUserId: string) => {
/* In this function whether you changed the order of the values when passing as parameters
it will always return only one id using localeCompare */
const comp = currentUserId.localeCompare(guestUserId);
if (comp === 0) {
return null;
}
if (comp === -1) {
return currentUserId + '_' + guestUserId;
} else {
return guestUserId + '_' + currentUserId;
}
}
Method 2
Use where clause with array-contains query for retrieving the chat document. And when creating add two user Ids to array and set the array with a relevant field name.
Firestore docs for querying arrays

MongoDB javascript query; filter for value in in a column array

Using vanilla javascript, I'm using Mongo's dpd.js for querying the db.
In this snippet, I am trying find all matches for "design" in the column roleConsiderations within the collection techniques.
The values in roleConsiderations are in arrays - ["development", "design", "content"] and all of my different attempt to query for "design" fails.
dpd.js query results in empty response:
var query = {"roleConsiderations": "design"};
dpd.techniques.get(query, function (result) {
console.log(result);
});
Attempting to query through the url only returns exact matches (where "design" is the only value in the array).
http://my.mongo.db/techniques?roleConsiderations=["design"]
So how to query a mongodb column filtering for a value in an array?

You can use $in operator to specify the elements you want the returned objects to contain in specified field.
const query = { roleConsiderations: { $in: [ 'design' ] } };
Horrible syntax, I know, but it should work.
For more reading, please refer to this.

CouchDB query issues

I will start off by saying while I am not new to CouchDB, I am new to querying the views using JavaScript and the web.
I have looked at multiple other questions on here, including CouchDB - Queries with params, couchDB queries, Couchdb query with AND operator, CouchDB Querying Dates, and Basic CouchDB Queries, just to list a few.
While all have good information in them, I haven't found one that has my particular problem in it.
I have a view set up like so:
function (docu) {
if(docu.status && docu.doc && docu.orgId.toString() && !docu.deleted){
switch(docu.status){
case "BASE":
emit(docu.name, docu);
break;
case "AIR":
emit(docu.eta, docu);
break;
case "CHECK":
emit(docu.checkTime, docu);
break;
}
}
}
with all documents having a status, doc, orgId, deleted, name, eta, and checkTime. (I changed doc to docu because of my custom doc key.
I am trying to query and emit based on a set of keys, status, doc, orgId, where orgId is an integer.
My jQuery to do this looks like so:
$.couch.db("myDB").view("designDoc/viewName", {
keys : ["status","doc",orgId],
success: function(data) {
console.log(data);
},
error: function(status) {
console.log(status);
}
});
I receive
{"total_rows":59,"offset":59,"rows":[
]}
Sometimes the offset is 0, sometimes it is 59. I feel I must be doing something wrong for this not to be working correctly.
So for my questions:
I did not mention this, but I had to set docu.orgId.toString() because I guess it parses the URL as a string, is there a way to use this number as a numeric value?
How do I correctly view multiple documents based on multiple keys, i.e. if(key1 && key2) emit(doc.name, doc)
Am I doing something obviously wrong that I lack the knowledge to notice?
Thank you all.

You're so very close. To answer your questions
When you're using docu.orgId.toString() in that if-statement you're basically saying: this value must be truthy. If you didn't convert to string, any number, other than 0, would be true. Since you are converting to a string, any value other than an empty string will be true. Also, since you do not use orgId as the first argument in an emit call, at least not in the example above, you cannot query by it at all.
I'll get to this.
A little.
The thing to remember is emit creates a key-value table (that's really all a view is) that you can use to query. Let's say we have the following documents
{type:'student', dept:'psych', name:'josh'},
{type:'student', dept:'compsci', name:'anish'},
{type:'professor', dept:'compsci', name:'kender'},
{type:'professor', dept:'psych', name:'josh'},
{type:'mascot', name:'owly'}
Now let's say we know that for this one view, we want to query 1) everything but mascots, 2) we want to query by type, dept, and name, all of the available fields in this example. We would write a map function like this:
function(doc) {
if (doc.type === 'mascot') { return; } // don't do anything
// allow for queries by type
emit(doc.type, null); // the use of null is explained below
// allow queries by dept
emit(doc.dept, null);
// allow for queries by name
emit(doc.name, null);
}
Then, we would query like this:
// look for all joshs
$.couch.db("myDB").view("designDoc/viewName", {
keys : ["josh"],
// ...
});
// look for everyone in the psych department
$.couch.db("myDB").view("designDoc/viewName", {
keys : ["psych"],
// ...
});
// look for everyone that's a professor and everyone named josh
$.couch.db("myDB").view("designDoc/viewName", {
keys : ["professor", "josh"],
// ...
});
Notice the last query isn't and in the sense of a logical conjunction, it's in the sense of a union. If you wanted to restrict what was returned to documents that were only professors and also joshs, there are a few options. The most basic would be to concatenate the key when you emit. Like
emit('type-' + doc.type + '_name-' + doc.name, null);
You would then query like this: key : ["type-professor_name-josh"]
It doesn't feel very proper to rely on strings like this, at least it didn't to me when I first started doing it, but it is a quite common method for querying key-value stores. The characters - and _ have no special meaning in this example, I simply use them as delimiters.
Another option would be what you mentioned in your comment, to emit an array like
emit([ doc.type, doc.name ], null);
Then you would query like
key: ["professor", "josh"]
This is perfectly fine, but generally, the use case for emitting arrays as keys, is for aggregating returned rows. For example, you could emit([year, month, day]) and if you had a simple reduce function that basically passed the records through:
function(keys, values, rereduce) {
if (rereduce) {
return [].concat.apply([], values);
} else {
return values;
}
}
You could query with the url parameter group_level set to 1 or 2 and start querying by year and month or just year on the exact same view using arrays as keys. Compared to SQL or Mongo it's mad complicated and convoluted, but hey, it's there.
The use of null in the view is really for resource saving. When you query a view, the rows contain an _id that you can use in a second ajax call to get all the documents from, for example, _all_docs.
I hope that makes sense. If you need any clarification you can use the comments and I'll try my best.

MongoDB search string in array of objects

I have a structure in MongoDB that have different amounts of items in an array called "items". To make the search, I am using the following command, which first turns the contents into a string, as in this.items there is a different structure depending on the object:
db.getCollection('docs').find.('JSON.stringify(this.items[0].value).toLowerCase().indexOf("text")!=-1')
My problem is that as I do not know the amount of items that each document has, I would have to use a wildcard as this.items[*].value, but it does not work.
Does anyone know any solution, or have another idea for this?

You can use the $elemMatch (https://docs.mongodb.com/manual/reference/operator/projection/elemMatch/)
db.docs.find({items: {$elemMatch: {value: {$regex : "text"}}}});
So this query will find all documents with an item in the items array that contain the string "text" in the value property, after this operation you can count how much items the document has.

You can use dot notation of items.value to target the value field of all items elements, and a regular expression to perform the case-insensitive sub-string match:
db.getCollection('docs').find({ 'items.value': /text/i })

You can iterate each document and apply the indexOf, something like this..
var cursor = db.getCollection('docs').find({}); // get all docs
var newOut = []; // new array of items if match with some condition
while ( cursor.hasNext() ){ // iterate all docs
var doc = cursor.next(); // get the document in focus
doc.items.forEach(function(item){ // iterate the items of doc.items
if ( item.toLowerCase().indexOf("text") !== -1 ) // check if text exists in array
newOut.push(item); // add to new array
});
};
printjson(newOut);

node.js, mongo quick way to hold unique number

I want to let mongo hold an incrementing number for me such that I can call that number and then generate a string from it.
x = 1e10
x.toString(36).substring(2,7)
>>'dqpds'
I have a way to increment the number every time I call it from mongo
db.counter.update({ _id: 1 }, { $inc: { seq: 1 } }, {upsert: true},
function(err, val){
//...
})
But I want to set the number to something like 1e10 at the beginning such that I get a 5 character long string, But I would rather not have something more than one call to the database.
How to I set a default value for the upsert in mongo. Or do you have a more efficient way of generating a unique 5 - 6 character string?

If you only need a unique id which is not necessarily sequential, you can use the first part of ObjectId.
From the above document there is a description:
ObjectId is a 12-byte BSON type, constructed using:
a 4-byte timestamp,
a 3-byte machine identifier,
a 2-byte process id, and
a 3-byte counter, starting with a random value.
So you can do like this:
x = ObjectId().toString().subString(0,4)
This approach doesn't involve database IO, so the performance would be better. If you want to be more sure about its uniqueness, add the last 2 bytes of the counter to make a 6 character one.

There is a way to do this in MongoDB.
You use the findAndModify command and it's described in detail in exactly the context you are looking for here:
http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/