Example of Flask admin making use of foreign keys and image upload. Just for illustrative proposes!

app.py

from flask import Flask, url_for
from flask_admin import Admin, form
from flask_sqlalchemy import SQLAlchemy
from flask_admin.contrib.sqla import ModelView
from jinja2 import Markup

"""
This just an illustrative example, take it as a concept example, 
nothing more, it is not secure and does not handle image deletion. 
Hereby more complete examples: 
https://github.com/flask-admin/flask-admin/blob/master/examples/forms/app.py
https://gist.github.com/jrm2k6/8905880
"""

app = Flask(__name__, static_folder='static', static_url_path='/static')
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.secret_key = b'Your App Secret!!!'

db = SQLAlchemy(app)

class User(db.Model):
    id =  db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(50), nullable=False, unique=True)
    name = db.Column(db.String(255), nullable=False)
    surname = db.Column(db.String(255), nullable=False)
    email = db.Column(db.String(120), nullable=False, unique=True)

    def __repr__(self):
        return f'<User {self.username}>'

    def __str__(self):
        return self.username


class Image(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(55), nullable=False, unique=True)
    path = db.Column(db.String(255))

    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    user = db.relationship('User', backref='user.thumbinail', lazy=True)

    def __str__(self):
        src = url_for('static', filename=self.path)
        # Markup not working...!?!?!?!!
        return Markup(f'<img src={src} height=12px> <strong>{self.title}</strong>')

class ImageView(ModelView):
    def _list_thumbnail(view, context, model, name):
        if not model.path:
            return ''

        return Markup('<img src="%s">' % url_for('static',
            filename=form.thumbgen_filename(model.path)))

    column_formatters = { 'path': _list_thumbnail }

    # Alternative way to contribute field is to override it completely.
    # In this case, Flask-Admin won't attempt to merge various parameters for the field.
    form_extra_fields = {
        'path': form.ImageUploadField('Image',
                                      base_path=app.static_folder,
                                      thumbnail_size=(100, 100, True))
    }
    
    
class ImageView(ModelView):
    def _list_thumbnail(view, context, model, name):
        if not model.path:
            return ''

        return Markup('<img src="%s">' % url_for('static',
            filename=form.thumbgen_filename(model.path)))

    column_formatters = { 'path': _list_thumbnail }

    # Alternative way to contribute field is to override it completely.
    # In this case, Flask-Admin won't attempt to merge various parameters for the field.
    form_extra_fields = {
        'path': form.ImageUploadField('Image',
                                      base_path=app.static_folder,
                                      thumbnail_size=(100, 100, True))
    }


class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(255), nullable=False)
    text = db.Column(db.Text)

    author_id = db.Column(db.Integer, db.ForeignKey('user.id'),
            nullable=False)
    author = db.relationship('User', backref=db.backref('posts'), lazy=True)

    def __repr__(self):
        return f'<Post: "ID":{self.id}, "TITLE":{self.title}>'

    def __str__(self):
        return self.title

db.init_app(app)
db.create_all()

# Add administrative views here
admin = Admin(app, name='microblog', template_mode='bootstrap3')
admin.add_view(ModelView(User, db.session))
admin.add_view(ModelView(Post, db.session))
admin.add_view(ImageView(Image, db.session))

if __name__=="__main__":
    app.run(debug=True)