Collapse to HasOneThrough

models.ts

import { Model, InferAttributes, InferCreationAttributes, DataTypes, CreationOptional, NonAttribute } from '@sequelize/core';
import { Attribute, PrimaryKey, NotNull, Table, AutoIncrement, BelongsToMany, AfterFind } from '@sequelize/core/decorators-legacy';

export class PictureSignature extends Model<InferAttributes<PictureSignature>, InferCreationAttributes<PictureSignature>> {
    @Attribute(DataTypes.DATE(6))
    declare createdAt: CreationOptional<Date>;

    @Attribute(DataTypes.INTEGER)
    @PrimaryKey
    @NotNull
    declare personId: number;

    @Attribute(DataTypes.INTEGER)
    @PrimaryKey
    @NotNull
    declare pictureId: number;
}

@Table({ tableName: 'Persons' })
export class Person extends Model<InferAttributes<Person>, InferCreationAttributes<Person>> {
    @Attribute(DataTypes.INTEGER)
    @AutoIncrement
    @PrimaryKey
    declare id: CreationOptional<number>;

    @Attribute(DataTypes.TEXT)
    declare name: string;
}

@Table({ tableName: 'Pictures' })
export class Picture extends Model<InferAttributes<Picture>, InferCreationAttributes<Picture>> {
    @Attribute(DataTypes.INTEGER)
    @AutoIncrement
    @PrimaryKey
    declare id: CreationOptional<number>;

    @BelongsToMany(() => Person, {
        through: () => PictureSignature,
        foreignKey: 'pictureId',
        otherKey: 'personId',
    })
    declare signers?: NonAttribute<Person[]>;

    // derived single
    declare firstSigner?: NonAttribute<Person>;

    @AfterFind
    static collapseFirstSigner(rows: Picture | Picture[] | null): void {
        const pictures = Array.isArray(rows) ? rows : rows ? [rows] : [];
        for (const pic of pictures) {
            const signers = pic.signers ?? [];
            if (!signers.length) continue;

            // pick signer with smallest createdAt on the join row
            const earliest = signers.reduce<Person | undefined>((min, person) => {
                const curCreatedAt = (person as any).PictureSignature?.createdAt ?? Number.POSITIVE_INFINITY;
                const minCreatedAt = min ? ((min as any).PictureSignature?.createdAt ?? Number.POSITIVE_INFINITY) : Number.POSITIVE_INFINITY;
                return curCreatedAt < minCreatedAt ? person : min;
            }, undefined);

            // store it as a derived single field
            // @ts-expect-error setDataValue exists on Model
            pic.setDataValue('firstSigner', earliest);
        }
    }
}

query.ts

const pictures = await Picture.findAll({
  include: [{
    association: 'signers',
    through: { attributes: ['createdAt'] }, // must include createdAt from join table
  }],
});

for (const pic of pictures) {
  console.log(pic.firstSigner?.name);
}