我的flask应用程序中具有以下SQLAlchemy模型：

class User(UserMixin,db.Model):
    __tablename__ = "users"

    # relationships
    instruments = db.relationship(
        Instrument,backref="instruments",passive_deletes=True,lazy="dynamic"
    )
    transactions = db.relationship(
        Transaction,backref="transactions",lazy="dynamic"
    )
    round_trips = db.relationship(
        RoundTrip,backref="round_trips",lazy="dynamic"
    )

    # columns
    id = db.Column(db.Integer,primary_key=True)
    username = db.Column(db.String(24),unique=True,index=True)

class Instrument(db.Model):
    __tablename__ = "instruments"

    # relationships
    user = db.relationship("User")
    transaction = db.relationship(
        Transaction,uselist=False,backref="transaction",passive_deletes=True
    )

    # columns
    id = db.Column(db.Integer,primary_key=True)
    user_id = db.Column(
        db.Integer,db.ForeignKey("users.id",onupdate="CASCADE",ondelete="CASCADE"),index=True,nullable=False,)
    symbol = db.Column(db.String(100),index=True)

class Transaction(db.Model):
    __tablename__ = "transactions"

    # relationships
    user = db.relationship("User")
    instrument = db.relationship("Instrument")
    transaction_round_trip_mapping = db.relationship(
        "RoundTripTransactionmapping",backref="round_trip_mappings",lazy="dynamic",)

    # columns
    id = db.Column(db.Integer,)
    instrument_id = db.Column(
        db.Integer,db.ForeignKey("instruments.id",)
    amount = db.Column(db.Float,nullable=False)

class RoundTrip(ResourceMixin,db.Model):
    __tablename__ = "round_trips"

    # relationships
    user = db.relationship("User")
    transaction_round_trip_mapping = db.relationship(
        "RoundTripTransactionmapping",backref="transaction_mappings",)

class RoundTripTransactionmapping(db.Model):
    __tablename__ = "round_trip_transaction_mappings"

    # relationships
    round_trip = db.relationship("RoundTrip")
    transaction = db.relationship("Transaction")

    # columns
    id = db.Column(db.Integer,primary_key=True)
    round_trip_id = db.Column(
        db.Integer,db.ForeignKey("round_trips.id",)
    transaction_id = db.Column(
        db.Integer,db.ForeignKey("transactions.id",)
    closing_transaction = db.Column(db.Boolean,nullable=False)

一个User可以有一个或多个Transaction。 User可以有一个或多个Instrument。 Transaction可以有一个Instrument。一个RoundTrip可以有一个或多个Transaction。

RoundTripTransactionmapping模型只是一个映射表，用于将RoundTrip绑定到一个或多个Transactions。

我想用RoundTrip查询所有Instrument.symbol == "GOOG"。

如何使用SQLAlchemy的ORM做到这一点？

（我什至还没有正确定义这些关系？对此我有点迷茫。）