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When a relationship contains an array of related documents, foreign filters enable precise filtering with AND logic across items, unlike normal filters which use OR logic.

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Array filter logic

Normal filters use OR logic across array items:
  • Find films where actors.country = "France" OR actors.age > 35
  • Returns films with ANY actor who is French OR over 35 (possibly different actors)
Foreign filters use AND logic across array items:
  • Find films where _foreign(actors, country = "France" AND age > 35)
  • Returns films with an actor who is BOTH French AND over 35 (same actor meets both conditions)

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Why this matters

Precise filtering is essential when you need to find documents where a related item meets multiple conditions simultaneously. Without this capability, you’d need to denormalize data or post-process results in your application code.

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Film cast example

Consider a film with three actors:
Where the actors are:

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Query 1: Normal filter (OR logic)

Result: Film is included because:
  • Actor B is French (country = “France”) ✓
  • Actor A is over 35 (age > 35) ✓
Both conditions are met, but by different actors.

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Query 2: Foreign filter (AND logic)

Result: Film is included because:
  • Actor B is BOTH French AND over 35 ✓
A single actor meets both conditions.

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Real-world examples

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Find products with multiple required certifications

Find products that have both ISO 9001 and CE certifications issued after 2020:

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Find articles with specific tag combinations

Find articles that have both the “security” AND “encryption” tags (same article, not different articles):
Wait, this won’t work as expected because a single tag can’t have multiple names. For this use case, you’d want to check if an article has both tags, which would require a different approach (checking two separate arrays or normalizing differently).

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Find employees with specific skill levels

Find employees with both Python AND JavaScript at advanced level:

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When to use precise filtering

Use foreign filters with AND logic when:
  • A document has an array of related items
  • You need to find items where a single related item meets multiple conditions
  • You’re filtering on nested properties of array items
Use normal filters with OR logic when:
  • You’re checking if any item in an array matches your criteria
  • You need broader matching across multiple array items

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Next steps

Foreign filters overview

Learn about foreign filters and filtering by joined data

Define relationships

Set up the relationships this filtering depends on