Flask-SQLAlchemy，如何在多对多关系中对结果进行排序

Question

我正在使用flask-sqlalchemy。

我想在多对多关系中添加一个 ORDER 列。

我的模型：

image_category_association = db.Table('category_image_relation',
    db.Column('id', db.Integer, primary_key=True),
    db.Column('category_id', db.Integer, db.ForeignKey('image_category.category_id')),
    db.Column('image_id', db.Integer, db.ForeignKey('image.image_id')),
    db.Column('order', db.Integer, unique=True, autoincrement=True)
)
class ImageCategory(db.Model):
    __bind_key__ = 'ads'
    __tablename__ = 'image_category'
    category_id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(255), default='')
    add_time = db.Column(db.Integer, default=0)
    images = db.relationship('Image', secondary=image_category_relation, backref='categories', lazy='dynamic')

class Image(db.Model):
    __bind_key__ = 'ads'
    __tablename__ = 'image'
    image_id = db.Column(db.Integer, primary_key=True)
    add_time = db.Column(db.Integer, default=0)
    hash = db.Column(db.String(64), default='')
    url = db.Column(db.String(255), default='')
category = ImageCategory.query.get(1)
category.images.append(image1, image2, image3)
images = category.images.all()  # order by the ORDER column

是否可以更新表 image_category_association 中的 ORDER 列值？

以及我如何按 ORDER 列对类别.images.all（） 排序。

Answer 1

使用关联对象 - 请参阅下面的自包含示例（我省略了一些字段）。订单在 ImageCategoryToImageAssociation 类中创建/更新 - 请参阅 build_db 方法以了解它的使用方式。

另请注意定义db.relationship ImageCategory images 属性中的order_by="ImageCategoryToImageAssociation.order" - category_1的图像按顺序插入，但查询数据库按排序顺序返回它们。

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy
app = Flask(__name__)
app.config['DEBUG'] = True
app.config['SECRET_KEY'] = 'super-secret'
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['SQLALCHEMY_ECHO'] = True
db = SQLAlchemy(app)

class ImageCategoryToImageAssociation(db.Model):
    __tablename__ = 'image_categories_to_image_association'
    image_category_id = db.Column(db.Integer, db.ForeignKey('image_categories.id'), primary_key=True)
    image_id = db.Column(db.Integer, db.ForeignKey('images.id'), primary_key=True)
    order = db.Column(db.Integer, default=0)
    image = db.relationship("Image", back_populates="categories")
    category = db.relationship("ImageCategory", back_populates="images")

class ImageCategory(db.Model):
    __tablename__ = 'image_categories'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(255), unique=True)
    images = db.relationship(ImageCategoryToImageAssociation, back_populates="category", order_by="ImageCategoryToImageAssociation.order")

class Image(db.Model):
    __tablename__ = 'images'
    id = db.Column(db.Integer, primary_key=True)
    url = db.Column(db.String(255), default='')
    categories = db.relationship(ImageCategoryToImageAssociation, back_populates="image")

@app.route('/')
def index():
    _html = ""
    categories = ImageCategory.query.all()
    for category in categories:
        _html += "<h4>Category : {category}</h4>".format(category=category.name)
        _html += "<ul>"
        for assoc in category.images:
            _html += "<li>Image : {url}; Order: {order}</li>".format(url=assoc.image.url, order=assoc.order)
        _html += "</ul>"
    return _html

def build_db():
    db.drop_all()
    db.create_all()
    # Add Categories
    category_1 = ImageCategory(name="First Category")
    db.session.add(category_1)
    category_2 = ImageCategory(name="Second Category")
    db.session.add(category_2)
    # Add Images
    image_1 = Image(url="First Image")
    db.session.add(image_1)
    image_2 = Image(url="Second Image")
    db.session.add(image_2)
    image_3 = Image(url="Third Image")
    db.session.add(image_3)
    # Associate images with category 1
    a_1 = ImageCategoryToImageAssociation(order=0)
    a_1.image = image_1
    category_1.images.append(a_1)
    a_2 = ImageCategoryToImageAssociation(order=2)
    a_2.image = image_2
    category_1.images.append(a_2)
    a_3 = ImageCategoryToImageAssociation(order=1)
    a_3.image = image_3
    category_1.images.append(a_3)
    # Associate images with category 2 in reverse order
    a_1 = ImageCategoryToImageAssociation(order=2)
    a_1.image = image_1
    category_2.images.append(a_1)
    a_2 = ImageCategoryToImageAssociation(order=1)
    a_2.image = image_2
    category_2.images.append(a_2)
    a_3 = ImageCategoryToImageAssociation(order=0)
    a_3.image = image_3
    category_2.images.append(a_3)
    db.session.commit()

@app.before_first_request
def before_first_request():
    build_db()
if __name__ == '__main__':
    app.run()