36 lines
1.2 KiB
Python
36 lines
1.2 KiB
Python
"""商品中心服务：多单位换算等。
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约定：
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- Product.base_unit 为基本单位；UnitConversion.rate 表示 1 个录入单位 = rate × 基本单位
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- 单据行 quantity 恒存基本单位数量；录入单位/数量存 source_unit/source_quantity
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"""
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from __future__ import annotations
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from decimal import Decimal
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from .models import Product, UnitConversion
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class UnitConversionNotFound(Exception):
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"""商品缺少该单位的换算率。"""
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def to_base(product: Product, unit, quantity: Decimal) -> Decimal:
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"""把"录入单位数量"换算成基本单位数量。
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unit 可以是 Unit 实例、pk 或 code；与基本单位相同（或未传）时原样返回。
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"""
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if unit in (None, ""):
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return Decimal(str(quantity))
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base = product.base_unit
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if base is not None and (unit == base or getattr(unit, "pk", None) == base.pk):
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return Decimal(str(quantity))
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conv = UnitConversion.objects.filter(product=product, unit=unit).first()
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if conv is None:
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unit_code = getattr(unit, "code", str(unit))
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raise UnitConversionNotFound(
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f"product {product.code} has no conversion for unit {unit_code}"
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)
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return Decimal(str(quantity)) * conv.rate
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