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Storing multiple values into a single field in mysql database that preserve order in Django

I've been trying to build a Tutorial system that we usually see on websites. Like the ones we click next -> next -> previous etc to read.
All Posts are stored in a table(model) called Post. Basically like a pool of post objects.
Post.objects.all() will return all the posts.
Now there's another Table(model)
called Tutorial That will store the following,
class Tutorial(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
tutorial_heading = models.CharField(max_length=100)
tutorial_summary = models.CharField(max_length=300)
series = models.CharField(max_length=40) # <---- Here [10,11,12]
...
Here entries in this series field are post_ids stored as a string representation of a list.
example: series will have [10,11,12] where 10, 11 and 12 are post_id that correspond to their respective entries in the Post table.
So my table entry for Tutorial model looks like this.
id heading summary series
"5" "Series 3 Tutorial" "lorem on ullt consequat." "[12, 13, 14]"
So I just read the series field and get all the Posts with the ids in this list then display them using pagination in Django.
Now, I've read from several stackoverflow posts that having multiple entries in a single field is a bad idea. And having this relationship to span over multiple tables as a mapping is a better option.
What I want to have is the ability to insert new posts into this series anywhere I want. Maybe in the front or middle. This can be easily accomplished by treating this series as a list and inserting as I please. Altering "[14,12,13]" will reorder the posts that are being displayed.
My question is, Is this way of storing multiple values in field for my usecase is okay. Or will it take a performance hit Or generally a bad idea. If no then is there a way where I can preserve or alter order by spanning the relationship by using another table or there is an entirely better way to accomplish this in Django or MYSQL.

Here entries in this series field are post_ids stored as a string representation of a list.
(...)
So I just read the series field and get all the Posts with the ids in this list then display them using pagination in Django.
DON'T DO THIS !!!
You are working with a relational database. There is one proper way to model relationships between entities in a relational database, which is to use foreign keys. In your case, depending on whether a post can belong only to a single tutorial ("one to many" relationship) or to many tutorials at the same time ("many to many" relationship, you'll want either to had to post a foreign key on tutorial, or to use an intermediate "post_tutorials" table with foreign keys on both post and tutorials.
Your solution doesn't allow the database to do it's job properly. It cannot enforce integrity constraints (what if you delete a post that's referenced by a tutorial ?), it cannot optimize read access (with proper schema the database can retrieve a tutorial and all it's posts in a single query) , it cannot follow reverse relationships (given a post, access the tutorial(s) it belongs to) etc. And it requires an external program (python code) to interact with your data, while with proper modeling you just need standard SQL.
Finally - but this is django-specific - using proper schema works better with the admin features, and with django rest framework if you intend to build a rest API.
wrt/ the ordering problem, it's a long known (and solved) issue, you just need to add an "order" field (small int should be enough). There are a couple 3rd part django apps that add support for this to both your models and the admin so it's almost plug and play.
IOW, there are absolutely no good reason to denormalize your schema this way and only good reasons to use proper relational modeling. FWIW I once had to work on a project based on some obscure (and hopefully long dead) PHP cms that had the brillant idea to use your "serialized lists" anti-pattern, and I can tell you it was both a disaster wrt/ performances and a complete nightmare to maintain. So do yourself and the world a favour: don't try to be creative, follow well-known and established best practices instead, and your life will be much happier. My 2 cents...

I can think of two approaches:
Approach One: Linked List
One way is using linked list like this:
class Tutorial(models.Model):
...
previous = models.OneToOneField('self', null=True, blank=True, related_name="next")
In this approach, you can access the previous Post of the series like this:
for tutorial in Tutorial.objects.filter(previous__isnull=True):
print(tutorial)
while(tutorial.next_post):
print(tutorial.next)
tutorial = tutorial.next
This is kind of complicated approach, for example whenever you want to add a new tutorial in middle of a linked-list, you need to change in two places. Like:
post = Tutorial.object.first()
next_post = post.next
new = Tutorial.objects.create(...)
post.next=new
post.save()
new.next = next_post
new.save()
But there is a huge benefit in this approach, you don't have to create a new table for creating series. Also, there is possibility that the order in tutorials will not be modified frequently, which means you don't need to take too much hassle.
Approach Two: Create a new Model
You can simply create a new model and FK to Tutorial, like this:
class Series(models.Model):
name = models.CharField(max_length=255)
class Tutorial(models.Model):
..
series = models.ForeignKey(Series, null=True, blank=True, related_name='tutorials')
order = models.IntegerField(default=0)
class Meta:
unique_together=('series', 'order') # it will make sure that duplicate order for same series does not happen
Then you can access tutorials in series by:
series = Series.object.first()
series.tutorials.all().order_by('tutorials__order')
Advantage of this approach is its much more flexible to access Tutorials through series, but there will be an extra table created for this, and one extra field as well to maintain order.

How to implement composition/agregation with NDB on GAE

How do we implement agregation or composition with NDB on Google App Engine ? What is the best way to proceed depending on use cases ?
Thanks !
I've tried to use a repeated property. In this very simple example, a Project have a list of Tag keys (I have chosen to code it this way instead of using StructuredProperty because many Project objects can share Tag objects).
class Project(ndb.Model):
name = ndb.StringProperty()
tags = ndb.KeyProperty(kind=Tag, repeated=True)
budget = ndb.FloatProperty()
date_begin = ndb.DateProperty(auto_now_add=True)
date_end = ndb.DateProperty(auto_now_add=True)
#classmethod
def all(cls):
return cls.query()
#classmethod
def addTags(cls, from_str):
tagname_list = from_str.split(',')
tag_list = []
for tag in tagname_list:
tag_list.append(Tag.addTag(tag))
cls.tags = tag_list
--
Edited (2) :
Thanks. Finally, I have chosen to create a new Model class 'Relation' representing a relation between two entities. It's more an association, I confess that my first design was unadapted.

An alternative would be to use BigQuery. At first we used NDB, with a RawModel which stores individual, non-aggregated records, and an AggregateModel, which a stores the aggregate values.
The AggregateModel was updated every time a RawModel was created, which caused some inconsistency issues. In hindsight, properly using parent/ancestor keys as Tim suggested would've worked, but in the end we found BigQuery much more pleasant and intuitive to work with.
We just have cronjobs that run everyday to push RawModel to BigQuery and another to create the AggregateModel records with data fetched from BigQuery.
(Of course, this is only effective if you have lots of data to aggregate)

It really does depend on the use case. For small numbers of items StructuredProperty and repeated properties may well be the best fit.
For large numbers of entities you will then look at setting the parent/ancestor in the Key for composition, and have a KeyProperty pointing to the primary entity in a many to one aggregation.
However the choice will also depend heavily on the actual use pattern as well. Then considerations of efficiency kick in.
The best I can suggest is consider carefully how you plan to use these relationships, how active are they (ie are they constantly changing, adding, deleting), do you need to see all members of the relation most of the time, or just subsets. These consideration may well require adjustments to the approach.

Is querying NDB JsonProperty in Google App Engine possible? If not, any alternatives?

Is there any way of using JsonProperties in queries in NDB/GAE? I can't seem to find any information about this.
Person.query(Person.custom.eye_color == "blue").fetch()
With a model looking something like this:
class Person(ndb.Model):
height = ndb.IntegerProperty(default=-1)
#...
#...
custom = ndb.JsonProperty(indexed=False, compressed=False)
The use case is this: I'm storing data about customers, where we at first only needed to query specific data. Now, we want to be able to query for any type of registred data about the persons. For example eye color, which some may have put into the system, or any other custom key/value pair in our JsonProperty.
I know about the expando class but for me, it seems a lot easier to be able to query jsonproperty and to keep all the custom properties on the same "name"; custom. That means that the front end can just loop over the properties in custom. If an expando class would be used, it would be harder to differentiate.

Rather than using a JSONProperty have you considered using a StructuredProperty. You maintain the same structure, just stored differently and you can filter by sub components of the StructureProperty with some restrictions, but that may be sufficient.
See https://developers.google.com/appengine/docs/python/ndb/queries#filtering_structured_properties
for querying StructuredProperties.

Query entities wih a specific parent's property in GAE and python?

Is it possible in any way to query entities using one of their parent's property in GAE, like this (which doesn't work)?
class Car(db.Model):
title = db.StringProperty()
type = db.StringProperty()
class Part(db.Model):
title = db.StringProperty()
car = Car()
car.title = 'BMW X5'
car.type = 'SUV'
car.put()
part = Part(parent = car)
part.title = 'Left door'
part.put()
parts = Part.all()
parts.filter('parent.type ==', 'SUV') # this in particular
I've read about ReferenceProperty, and Indexes but I'm not sure what I need.
GAE lets me set a parent to the Part entity, but do I need an actually (kind of duplicate):
parent = db.ReferenceProperty(Car, required=True)
That would feel like duplicating what the system does already since it has a parent. Or is there an other way?

It's not an answer to your question as such, but NDB offers structured properties.
https://developers.google.com/appengine/docs/python/ndb/properties#structured
You can structure a model's properties. For example, you can define a model class Contact containing a list of addresses, each with internal structure.
Although the structured properties instances are defined using the same syntax as for model classes, they are not full-fledged entities. They don't have their own keys in the Datastore. They cannot be retrieved independently of the entity to which they belong. An application can, however, query for the values of their individual fields.
So here car would contain parts as a structured property. If this is viable in your use case depends on how you structure your data. If you want to know what parts make up a specific car, that seems viable. If you want to filer global parts regardless of what car they belong to, then you can still do that but you'll have to make the "parts" inside each car also refer to a different model. If you see what I mean (I'm not sure I do), as each car contains it's own parts.

Adding the parent as an explicit property isn't going to help.
You can break it up in two parts though:
for suv in Car.all().filter('type', 'SUV'):
for part in Part.all(ancestor=suv):
...do something with part...

If you want to query on the property of another (parent) object, you gotta get that object first.
I can think of two solutions to your problem:
Guido's way is to query for the parent, and then query for the part. This way issues more queries.
The second way is to store a copy of parent.type inside your Part. The downsides are that you're storing duplicate data (more storage), and you have to be careful that your the data in Part and data in Car match up. However, you only need to issue one query.
You'll have to figure out which one works better for you.

How to update a model

Assume I have a model definition like this:
class Image(db.Model):
id = db.StringProperty()
url = db.URLProperty()
Now I want to add some fields to this model to make it look like this:
class Image(db.Model):
id = db.StringProperty()
url = db.URLProperty()
width = db.IntegerProperty()
height = db.IntegerProperty()
So, this new model will be applied properly to newly added Image entities. But I also want to update already existing entities so that they contained these two new fields and fill them with values. Will an already existing entity get these two fields automatically so when I refer to them, it will give me empty fields or will it cause an error? I suppose I will have to create a helper function that will go through all existing entities and set new fields values, right? So, what should I keep in mind and how to better do this model update? I think it will happen sometimes as the application emerges, so I think it would be useful to have some straightforward flow to do that.

This exact scenario is covered in the GAE docs (articles section):
Updating your model's schema.
Basically just change the model definition as you've done, then perform some operation to supply default values for all your extant entities. There are several ways to do the second part - the article describes one.

No already exisiting entity won't get these two fields automatically or it won't assume it to None. It will cause an error when those fields are accessed in existing objects. Only solution avaliable now is to use remote_apy and write your own script to update the existing records. It won't be big deal, write a script to get all the records in the datastore and to set some default values for the new attributes..
Setting_Up_remote_api
Update_schema