DynamoDB: No way to combine multiple ConditionExpression

[SamStephens]

Describe the feature

According to https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.OperatorsAndFunctions.html it's possible to combine multiple conditions using AND and OR. However, from my reading of https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/dynamodb/table/update_item.html and https://boto3.amazonaws.com/v1/documentation/api/latest/reference/customizations/dynamodb.html#boto3.dynamodb.conditions.Attr, it is only possible to specify a single condition using the update_item operation.

Use Case

I have an attribute I want to update, but not if the update will decrease the attributes value. The attribute is not necessarily present. So I need my conditional expression to be attribute not exists OR attribute value < my value.

It appears the API supports this conditional expression, but not this SDK.

Proposed Solution

• Add a super class for boto3.dynamodb.conditions.Attr, maybe boto3.dynamodb.conditions.Condition.
• Make the ConditionExpression parameter a boto3.dynamodb.conditions.Condition.
• Give boto3.dynamodb.conditions.Condition three methods, And, Or, and Not, that each do the obvious thing.

Under this scheme, my ConditionExpression would be

ConditionExpression = Condition().Or(Attr(attribute_name).not_exists(), Attr(attribute_name).lt(updated_value))

Other Information

The only way I can see to workaround this is two conditional updates - firstly with not_exists, and then if the check fails because the attribute exists, try the update again with lt.

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

SDK version used

1.34.76

Environment details (OS name and version, etc.)

Ubuntu (Windows Subsystem for Linux)

[crh23]

Do the classes defined as follows fit your use-case?

boto3/boto3/dynamodb/conditions.py

Lines 158 to 238 in 096e458

	class ComparisonCondition(ConditionBase):
	expression_format = '{0} {operator} {1}'
	class Equals(ComparisonCondition):
	expression_operator = '='
	class NotEquals(ComparisonCondition):
	expression_operator = '<>'
	class LessThan(ComparisonCondition):
	expression_operator = '<'
	class LessThanEquals(ComparisonCondition):
	expression_operator = '<='
	class GreaterThan(ComparisonCondition):
	expression_operator = '>'
	class GreaterThanEquals(ComparisonCondition):
	expression_operator = '>='
	class In(ComparisonCondition):
	expression_operator = 'IN'
	has_grouped_values = True
	class Between(ConditionBase):
	expression_operator = 'BETWEEN'
	expression_format = '{0} {operator} {1} AND {2}'
	class BeginsWith(ConditionBase):
	expression_operator = 'begins_with'
	expression_format = '{operator}({0}, {1})'
	class Contains(ConditionBase):
	expression_operator = 'contains'
	expression_format = '{operator}({0}, {1})'
	class Size(ConditionAttributeBase):
	expression_operator = 'size'
	expression_format = '{operator}({0})'
	class AttributeType(ConditionBase):
	expression_operator = 'attribute_type'
	expression_format = '{operator}({0}, {1})'
	class AttributeExists(ConditionBase):
	expression_operator = 'attribute_exists'
	expression_format = '{operator}({0})'
	class AttributeNotExists(ConditionBase):
	expression_operator = 'attribute_not_exists'
	expression_format = '{operator}({0})'
	class And(ConditionBase):
	expression_operator = 'AND'
	expression_format = '({0} {operator} {1})'
	class Or(ConditionBase):
	expression_operator = 'OR'
	expression_format = '({0} {operator} {1})'
	class Not(ConditionBase):
	expression_operator = 'NOT'
	expression_format = '({operator} {0})'

You can write expressions like
ConditionExpression = Or(Attr(attribute_name).not_exists(), Attr(attribute_name).lt(updated_value))

I'm not sure if they are documented anywhere

[SamStephens]

@crh23 thanks, this is exactly what I needed.

I've closing this issue as support is present, and replacing it with an issue for the missing documentation.

[github-actions]

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