bcquality/microsoft/knowledge/data-modeling/tablerelation-field-length-must-match-related-field.md

bc-version	domain	keywords	technologies	countries	application-area
all	data-modeling	tablerelation field-length code text conditional-relation lookup	al	w1	all

A TableRelation field's length must match the related field

Description

TableRelation points a field at another table, or at a specific field of it with Table.Field. The relation is also used to validate entries and to drive the lookup. A referencing field that is shorter than the related field compiles without a length diagnostic: verified with AL compiler 30.0 and the CodeCop, UICop and PerTenantExtensionCop analyzers. Compare AL0685, which does warn about an analogous length mismatch for a FlowField's CalcFormula target.

A shorter field compiles cleanly and works for every value that happens to fit. It fails only when a real related value is longer than the field. For example, a Code[10] field related to a table with a Code[20] key fails when a user picks a 15-character code through the lookup. The runtime error reads "The length of the string is N, but it must be less than or equal to M characters."

The length Business Central's own relation check expects depends on whether the field has an unconditional relation:

• At least one unconditional relation, either a plain TableRelation = X or an unconditional branch: the field must have the exact length of the longest related field and the same type.
• Only conditional relations (if (...) X else if (...) Y): the field must be at least as long as the longest related field. A longer field is accepted, and a Code relation may be stored in a Text field.

The check is codeunit 134926 "Table Relation Test" in the BC test app. Its validation test is [Scope('OnPrem')], so it runs only on an on-premises test surface. It is not a compile-time guarantee. See table-relation-test-exclude-known-invalid-relations-via-event.md for how that check evaluates relations and how to exclude a known exception.

Best Practice

Before you add or change a TableRelation, read the declared type and length of every related field from its table definition. Lengths vary from table to table, so don't assume a typical Code[10] or Code[20]. Then size the field:

• For an unconditional relation, give it the same type and exact length as the related field.
• For an all-conditional relation, make it at least as long as the longest branch target.

When a tableextension adds a branch with modify(...), check the new target's length against the field's existing declaration too.

See sample: tablerelation-field-length-must-match-related-field.good.al.

Anti Pattern

A field whose declared Code/Text length is shorter than the field it relates to. One example is a Code[10] field with TableRelation to a table keyed on Code[20]. Another is a conditional relation where one branch targets a longer field than the declaration. Both compile without a diagnostic and fail at runtime once a real, longer value is used.

A field that is longer than its related field under an unconditional relation is a lesser deviation. It holds every valid value, but it fails Business Central's relation test and accepts values that can never satisfy the relation.

See sample: tablerelation-field-length-must-match-related-field.bad.al.

Related: transferfields-mirrored-fields-must-match-type-and-length.md covers the same length-mismatch failure between fields that TransferFields connects.

Source

• TableRelation property: syntax <TableName>[.<FieldName>], conditional IF ... ELSE relations, and use of the relation to validate entries.
• Compiler Warning (future error) AL0685: the analogous FlowField length diagnostic, which warns that the mismatch "could result in a runtime error".
• BCApps src/Layers/W1/Tests/Misc/TableRelationTest.Codeunit.al: line 46 says "Fields must have the exact length of the largest field they relate to". Lines 68-84 apply Field.Len < MaxRelatedFieldLength when every relation is conditional and Field.Len <> MaxRelatedFieldLength otherwise. Line 15 is [Scope('OnPrem')].
• BCApps src/Apps/W1/Subcontracting/Test/Tests/SubcCommentsAttachmentTest.Codeunit.al line 295 asserts the runtime text "The length of the string is 101".