cubicweb #636936 read_permission relation should not be mandatory [resolved]
We should be able to define Entity / Relation without a read permission (for use w/ internal session only). It's currently not possible because of the read_permission cardinality.
ValidationError: 1749 (read_permission): at least one relation can be read by is required on Entity type (1749
ValidationError: 1740 (read_permission): at least one relation can be read by is required on Relation type (1740)
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